In-depth analysis of the language structure echo in PHP (recommended)-PHP Tutorial-php.cn

Echo is actually a language structure that is often used when writing code in PHP. Therefore, the following article will give you a detailed analysis of the use of the language structure echo. Not much to say. , let’s go directly to the text.

1. Document (php.cn)

1.1 Output one or more strings

void echo ( string $arg1 [, string $... ] )

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1.2 Note

echo is not a function, but a PHP language structure, so you don’t have to use parentheses to specify parameters, single quotes or double quotes will do. .
echo does not behave like a function, so you cannot always use a function context.
When echo outputs multiple strings, parentheses cannot be used.
echo When short_open_tag is enabled in php.ini, there is a shortcut usage (view layer) = 'Hello World'; ?>
The main difference between echo and print is that echo accepts parameters list and has no return value.

1.3 Note

Note: Because it is a language constructor rather than a function, it cannot be called by variadic functions.

<?php

/**
 * Tip
 * 相对 echo 中拼接字符串而言，传递多个参数比较好，考虑到了 PHP 中连接运算符（“.”）的优先级。 传入多个参数，不需要圆括号保证优先级：
 */
echo "Sum: ", 1 + 2;
echo "Hello ", isset($name) ? $name : "John Doe", "!";

/** Tip
 * 如果是拼接的，相对于加号和三目元算符，连接运算符（“.”）具有更高优先级。为了正确性，必须使用圆括号：
 */
echo &#39;Sum: &#39; . (1 + 2);
echo &#39;Hello &#39; . (isset($name) ? $name : &#39;John Doe&#39;) . &#39;!&#39;;

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2、应用

2.1 输出基本数据类型

echo 123, &#39;abc&#39;, [12, 34];  // 123abcArray

echo "Sum: ", 1 + 2; // Sum: 3
echo &#39;Sum: &#39; . (1 + 2); // Sum: 3

echo "Hello ", isset($name) ? $name : "John Doe", "!"; // Hello John Doe!
echo &#39;Hello &#39; . (isset($name) ? $name : &#39;John Doe&#39;) . &#39;!&#39;; // Hello John Doe!

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2.2 输出对象类型

<?php 
class Customer {
    public function say() {
        return &#39;Hello World&#39;;
    }
}

echo (new Customer());

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Catchable fatal error: Object of class Customer could not be converted to string in /usercode/file.php on line 8

输出对象时汇报以上错误, 所以如果需要输出对象, 一定要在其内部实现 __toString()。

<?php 
class Customer {
    public function say() {
        return &#39;Hello World&#39;;
    }
    
    /**
     * __toString() 方法用于一个类被当成字符串时应怎样回应。例如 echo $obj; 应该显示些什么。此方法必须返回一个字符串，否则将发出一条 E_RECOVERABLE_ERROR 级别的致命错误。
     */
    public function __toString(){
        return $this->say();
    }
}

echo (new Customer()); // Hello World

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2.3 输出资源类型

echo tmpfile(); // Resource id #1

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3、源码

3.1 源码概述

php 是一门脚本语言, 所以所有的符号都会先经过词法解析和语法解析阶段, 这两个阶段由lex & yacc 完成。

在计算机科学里面，lex是一个产生词法分析器的程序。 Lex常常与yacc 语法分析器产生程序一起使用。Lex是许多UNIX系统的标准词法分析器产生程序，而且这个工具所作的行为被详列为POSIX标准的一部分。 Lex读进一个代表词法分析器规则的输入字符串流，然后输出以C语言实做的词法分析器源代码。

对应的文件在 Zend/zend_language_parser.y 和 Zend/zend_language_scanner.l。

3.2 字符转标记(Zend/zend_language_scanner.l)

<ST_IN_SCRIPTING>"echo" {
    RETURN_TOKEN(T_ECHO);
}

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ZEND引擎在读取一个PHP文件之后会先进行词法分析，就是用lex扫描，把对应的PHP字符转换成相应的标记（也叫token)，比如 echo $a; 在碰到这句首先会匹配到echo，符合上面的规则，然后就返回一个 T_ECHO 标记，这个在后面的语法分析会用上，也就是在 zend_language_parser.y 文件中

3.3 语法分析(Zend/zend_language_parser.y)

# %token Token就是一个个的“词块”
%token T_ECHO       "echo (T_ECHO)"

# statement T_ECHO echo_expr_list
statement:
        &#39;{&#39; inner_statement_list &#39;}&#39; { $$ = $2; }
    |    if_stmt { $$ = $1; }
    |    alt_if_stmt { $$ = $1; }
    |    T_WHILE &#39;(&#39; expr &#39;)&#39; while_statement
            { $$ = zend_ast_create(ZEND_AST_WHILE, $3, $5); }
    |    T_DO statement T_WHILE &#39;(&#39; expr &#39;)&#39; &#39;;&#39;
            { $$ = zend_ast_create(ZEND_AST_DO_WHILE, $2, $5); }
    |    T_FOR &#39;(&#39; for_exprs &#39;;&#39; for_exprs &#39;;&#39; for_exprs &#39;)&#39; for_statement
            { $$ = zend_ast_create(ZEND_AST_FOR, $3, $5, $7, $9); }
    |    T_SWITCH &#39;(&#39; expr &#39;)&#39; switch_case_list
            { $$ = zend_ast_create(ZEND_AST_SWITCH, $3, $5); }
    |    T_BREAK optional_expr &#39;;&#39;        { $$ = zend_ast_create(ZEND_AST_BREAK, $2); }
    |    T_CONTINUE optional_expr &#39;;&#39;    { $$ = zend_ast_create(ZEND_AST_CONTINUE, $2); }
    |    T_RETURN optional_expr &#39;;&#39;        { $$ = zend_ast_create(ZEND_AST_RETURN, $2); }
    |    T_GLOBAL global_var_list &#39;;&#39;    { $$ = $2; }
    |    T_STATIC static_var_list &#39;;&#39;    { $$ = $2; }
    |    T_ECHO echo_expr_list &#39;;&#39;        { $$ = $2; }
    |    T_INLINE_HTML { $$ = zend_ast_create(ZEND_AST_ECHO, $1); }
    |    expr &#39;;&#39; { $$ = $1; }
    |    T_UNSET &#39;(&#39; unset_variables &#39;)&#39; &#39;;&#39; { $$ = $3; }
    |    T_FOREACH &#39;(&#39; expr T_AS foreach_variable &#39;)&#39; foreach_statement
            { $$ = zend_ast_create(ZEND_AST_FOREACH, $3, $5, NULL, $7); }
    |    T_FOREACH &#39;(&#39; expr T_AS foreach_variable T_DOUBLE_ARROW foreach_variable &#39;)&#39;
        foreach_statement
            { $$ = zend_ast_create(ZEND_AST_FOREACH, $3, $7, $5, $9); }
    |    T_DECLARE &#39;(&#39; const_list &#39;)&#39;
            { zend_handle_encoding_declaration($3); }
        declare_statement
            { $$ = zend_ast_create(ZEND_AST_DECLARE, $3, $6); }
    |    &#39;;&#39;    /* empty statement */ { $$ = NULL; }
    |    T_TRY &#39;{&#39; inner_statement_list &#39;}&#39; catch_list finally_statement
            { $$ = zend_ast_create(ZEND_AST_TRY, $3, $5, $6); }
    |    T_THROW expr &#39;;&#39; { $$ = zend_ast_create(ZEND_AST_THROW, $2); }
    |    T_GOTO T_STRING &#39;;&#39; { $$ = zend_ast_create(ZEND_AST_GOTO, $2); }
    |    T_STRING &#39;:&#39; { $$ = zend_ast_create(ZEND_AST_LABEL, $1); }
;

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在 statement 看到了 T_ECHO, 后面跟着 echo_expr_list，再搜这个字符串，找到如下代码:

# echo_expr_list
echo_expr_list:
    echo_expr_list &#39;,&#39; echo_expr { $$ = zend_ast_list_add($1, $3); }
  | echo_expr { $$ = zend_ast_create_list(1, ZEND_AST_STMT_LIST, $1); }
;

echo_expr:
  expr { $$ = zend_ast_create(ZEND_AST_ECHO, $1); }
;

expr:
    variable              { $$ = $1; }
  | expr_without_variable { $$ = $1; }
;

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词法分析后得到单独存在的词块不能表达完整的语义，还需要借助规则进行组织串联。语法分析器就是这个组织者。它会检查语法、匹配Token，对Token进行关联。PHP7中，组织串联的产物就是抽象语法树（Abstract Syntax Tree，AST）, 详情请查看相关源码: 抽象语法树（Abstract Syntax Tree，AST）

这么看比较难理解，接下来我们从一个简单的例子看下最终生成的语法树。

$a = 123;
$b = "hi~";

echo $a,$b;

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具体解析过程这里不再解释，有兴趣的可以翻下zend_language_parse.y中，这个过程不太容易理解，需要多领悟几遍，最后生成的ast如下图：

In-depth analysis of the language structure echo in PHP (recommended)

4.4 模块初始化(main/main.c)

通过 write_function 绑定PHP输出函数php_output_wrapper 至 zend_utility_functions结构体, 此结构体会在xx被使用

# php_module_startup

zend_utility_functions zuf;

// ...

gc_globals_ctor();

zuf.error_function = php_error_cb;
zuf.printf_function = php_printf;
zuf.write_function = php_output_wrapper;
zuf.fopen_function = php_fopen_wrapper_for_zend;
zuf.message_handler = php_message_handler_for_zend;
zuf.get_configuration_directive = php_get_configuration_directive_for_zend;
zuf.ticks_function = php_run_ticks;
zuf.on_timeout = php_on_timeout;
zuf.stream_open_function = php_stream_open_for_zend;
zuf.printf_to_smart_string_function = php_printf_to_smart_string;
zuf.printf_to_smart_str_function = php_printf_to_smart_str;
zuf.getenv_function = sapi_getenv;
zuf.resolve_path_function = php_resolve_path_for_zend;
zend_startup(&zuf, NULL);

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zuf 是一个 zend_utility_functions 结构体，这样就把php_output_wrapper函数传给了zuf.write_function，后面还有好几层包装，最后的实现也是在main/main.c文件里面实现的，是下面这个函数：

/* {{{ php_output_wrapper
 */
static size_t php_output_wrapper(const char *str, size_t str_length)
{
    return php_output_write(str, str_length);
}

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在 php_out_wrapper 中调用的 php_output_write 在 main/output.c 中实现, 实现代码如下:

/* {{{ int php_output_write(const char *str, size_t len)
 * Buffered write 
 * #define PHP_OUTPUT_ACTIVATED        0x100000 
 * 当flags=PHP_OUTPUT_ACTIVATED，会调用sapi_module.ub_write输出, 每个SAPI都有自已的实现, cli中是调用sapi_cli_single_write()
 *  php_output_write(); //输出，有buffer, 调用php_output_op()
 *  php_output_write_unbuffered();//输出，没有buffer，调用PHP_OUTPUT_ACTIVATED，会调用sapi_module.ub_write
 *  php_output_set_status(); //用于SAPI设置output.flags
 *  php_output_get_status(); //获取output.flags的值
 */
PHPAPI size_t php_output_write(const char *str, size_t len)
{
    if (OG(flags) & PHP_OUTPUT_ACTIVATED) {
        php_output_op(PHP_OUTPUT_HANDLER_WRITE, str, len);
        return len;
    }
    if (OG(flags) & PHP_OUTPUT_DISABLED) {
        return 0;
    }
    return php_output_direct(str, len);
}
/* }}} */

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3.5 输出的终点(main/output.c fwrite函数)

不调用sapi_module的输出

static size_t (*php_output_direct)(const char *str, size_t str_len) = php_output_stderr;

static size_t php_output_stderr(const char *str, size_t str_len)
{
    fwrite(str, 1, str_len, stderr);
/* See http://support.microsoft.com/kb/190351 */
#ifdef PHP_WIN32
    fflush(stderr);
#endif
    return str_len;
}

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调用sapi_module的输出

sapi_module.ub_write(context.out.data, context.out.used);

if (OG(flags) & PHP_OUTPUT_IMPLICITFLUSH) {
    sapi_flush();
}

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php_output_op 详细实现如下:

/* {{{ static void php_output_op(int op, const char *str, size_t len)
 * Output op dispatcher, passes input and output handlers output through the output handler stack until it gets written to the SAPI 
 */
static inline void php_output_op(int op, const char *str, size_t len)
{
    php_output_context context;
    php_output_handler **active;
    int obh_cnt;

    if (php_output_lock_error(op)) {
        return;
    }

    php_output_context_init(&context, op);

    /*
     * broken up for better performance:
     *  - apply op to the one active handler; note that OG(active) might be popped off the stack on a flush
     *  - or apply op to the handler stack
     */
    if (OG(active) && (obh_cnt = zend_stack_count(&OG(handlers)))) {
        context.in.data = (char *) str;
        context.in.used = len;

        if (obh_cnt > 1) {
            zend_stack_apply_with_argument(&OG(handlers), ZEND_STACK_APPLY_TOPDOWN, php_output_stack_apply_op, &context);
        } else if ((active = zend_stack_top(&OG(handlers))) && (!((*active)->flags & PHP_OUTPUT_HANDLER_DISABLED))) {
            php_output_handler_op(*active, &context);
        } else {
            php_output_context_pass(&context);
        }
    } else {
        context.out.data = (char *) str;
        context.out.used = len;
    }

    if (context.out.data && context.out.used) {
        php_output_header();

        if (!(OG(flags) & PHP_OUTPUT_DISABLED)) {
#if PHP_OUTPUT_DEBUG
            fprintf(stderr, "::: sapi_write(&#39;%s&#39;, %zu)\n", context.out.data, context.out.used);
#endif
            sapi_module.ub_write(context.out.data, context.out.used);

            if (OG(flags) & PHP_OUTPUT_IMPLICITFLUSH) {
                sapi_flush();
            }

            OG(flags) |= PHP_OUTPUT_SENT;
        }
    }
    php_output_context_dtor(&context);
}

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以上了解了PHP输出函数的实现, 接下来了解echo实现.

3.6 输出动作的ZEND引擎实现(Zend/zend_vm_def.h)

ZEND_VM_HANDLER(40, ZEND_ECHO, CONST|TMPVAR|CV, ANY)
{
    USE_OPLINE
    zend_free_op free_op1;
    zval *z;

    SAVE_OPLINE();
    z = GET_OP1_ZVAL_PTR_UNDEF(BP_VAR_R);

    if (Z_TYPE_P(z) == IS_STRING) {
        zend_string *str = Z_STR_P(z);

        if (ZSTR_LEN(str) != 0) {
            zend_write(ZSTR_VAL(str), ZSTR_LEN(str));
        }
    } else {
        zend_string *str = _zval_get_string_func(z);

        if (ZSTR_LEN(str) != 0) {
            zend_write(ZSTR_VAL(str), ZSTR_LEN(str));
        } else if (OP1_TYPE == IS_CV && UNEXPECTED(Z_TYPE_P(z) == IS_UNDEF)) {
            GET_OP1_UNDEF_CV(z, BP_VAR_R);
        }
        zend_string_release(str);
    }

    FREE_OP1();
    ZEND_VM_NEXT_OPCODE_CHECK_EXCEPTION();
}

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可以看到在 zend vm 中通过调用zend_write来实现输出，接下来看下zend_write的实现。

3.7 zend_write实现(Zend/zend.c)

# Zend/zend.h
typedef int (*zend_write_func_t)(const char *str, size_t str_length);

# Zend/zend.c
ZEND_API zend_write_func_t zend_write;

# 如下图所示, zend_write的初始化是在zend_startup()函数里面，这是zend引擎启动的时候需要做的一些初始化工作，有下面一句：

zend_write = (zend_write_func_t) utility_functions->write_function; // php_output_wrapper

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zend_utility_functions *utility_functions 在 main/main.c php_module_startup()的 zuf中被定义:

zuf.write_function = php_output_wrapper;

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In-depth analysis of the language structure echo in PHP (recommended)

4、php(echo)加速

4.1 PHP echo真的慢么?

echo 输出大字符串(500K)的时候，执行时间会明显变长，所以会被认为PHP的echo性能很差, 实际上这并不是语言(PHP)问题, 而是一个IO问题(IO的速度限制了输出的速度)。

但是在某些时候echo执行时间过长, 会影响其他的服务, 进而影响整个系统。

那么使用 apache 时如何优化使的 echo 变快，让PHP的请求处理过程尽快结束？

4.2 还是可以优化的: 打开输出缓存

echo慢是在等待“写数据”成功返回，所以可打开输出缓存：

# 编辑php.ini
output_buffering = 4096 //bytes

# 调用ob_start()
ob_start();
echo $hugeString;
ob_end_flush();

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ob_start() 会开辟一块4096大小的buffer，所以如果 $hugeString 大于 4096，将不会起到加速作用。

echo 会立即执行成功返回, 因为数据暂时写到了我们的输出缓存中，如果buffer足够大，那么内容会等到脚本的最后，才一次性的发送给客户端(严格的说是发给webserver)。

5、输出时的类型转换

5.1输出时的类型转换规则

input	output	desc	code
Boolean	String	1 或 0	`echo true; // 1`
Integer	Integer	不转换	`echo 123; // 123`
Float	Float	不转换, 注意精度问题	`echo 123.234; // 123.234`
String	String	不转换	`echo 'abcd'; // abcd`
Array	Array	-	`echo [12, 34]; // Array`
Object	Catchable fatal error	Object of class stdClass could not be converted to string in file.php on line *	`echo json_decode(json_encode(['a' => 'b']));`
Resource	Resource id #1	-	`echo tmpfile(); // Resource id #1`
NULL	string	转为空字符串	`echo null; // 空字符串`

5.2 输出时的类型转换源码(Zend/zend_operators.h & Zend/zend_operators.c)

# Zend/zend_operators.h
ZEND_API zend_string* ZEND_FASTCALL _zval_get_string_func(zval *op);

# Zend/zend_operators.c
ZEND_API zend_string* ZEND_FASTCALL _zval_get_string_func(zval *op) /* {{{ */
{
try_again:
    switch (Z_TYPE_P(op)) {
        case IS_UNDEF:
        case IS_NULL:
        case IS_FALSE:
            return ZSTR_EMPTY_ALLOC();
        case IS_TRUE:
            if (CG(one_char_string)[&#39;1&#39;]) {
                return CG(one_char_string)[&#39;1&#39;];
            } else {
                return zend_string_init("1", 1, 0);
            }
        case IS_RESOURCE: {
            char buf[sizeof("Resource id #") + MAX_LENGTH_OF_LONG];
            int len;

            len = snprintf(buf, sizeof(buf), "Resource id #" ZEND_LONG_FMT, (zend_long)Z_RES_HANDLE_P(op));
            return zend_string_init(buf, len, 0);
        }
        case IS_LONG: {
            return zend_long_to_str(Z_LVAL_P(op));
        }
        case IS_DOUBLE: {
            return zend_strpprintf(0, "%.*G", (int) EG(precision), Z_DVAL_P(op));
        }
        case IS_ARRAY:
            zend_error(E_NOTICE, "Array to string conversion");
            return zend_string_init("Array", sizeof("Array")-1, 0);
        case IS_OBJECT: {
            zval tmp;
            if (Z_OBJ_HT_P(op)->cast_object) {
                if (Z_OBJ_HT_P(op)->cast_object(op, &tmp, IS_STRING) == SUCCESS) {
                    return Z_STR(tmp);
                }
            } else if (Z_OBJ_HT_P(op)->get) {
                zval *z = Z_OBJ_HT_P(op)->get(op, &tmp);
                if (Z_TYPE_P(z) != IS_OBJECT) {
                    zend_string *str = zval_get_string(z);
                    zval_ptr_dtor(z);
                    return str;
                }
                zval_ptr_dtor(z);
            }
            zend_error(EG(exception) ? E_ERROR : E_RECOVERABLE_ERROR, "Object of class %s could not be converted to string", ZSTR_VAL(Z_OBJCE_P(op)->name));
            return ZSTR_EMPTY_ALLOC();
        }
        case IS_REFERENCE:
            op = Z_REFVAL_P(op);
            goto try_again;
        case IS_STRING:
            return zend_string_copy(Z_STR_P(op));
        EMPTY_SWITCH_DEFAULT_CASE()
    }
    return NULL;
}
/* }}} */

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6、Zend/zend_compile.c对echo的解析

6.1 源码地址

PHP源码地址 zend_compile.h
PHP源码地址 zend_compile.c

6.2 zend_compile_expr 实现

# Zend/zend_compile.h
void zend_compile_expr(znode *node, zend_ast *ast);

# Zend/zend_compile.c
void zend_compile_expr(znode *result, zend_ast *ast) /* {{{ */
{
    /* CG(zend_lineno) = ast->lineno; */
    CG(zend_lineno) = zend_ast_get_lineno(ast);

    switch (ast->kind) {
        case ZEND_AST_ZVAL:
            ZVAL_COPY(&result->u.constant, zend_ast_get_zval(ast));
            result->op_type = IS_CONST;
            return;
        case ZEND_AST_ZNODE:
            *result = *zend_ast_get_znode(ast);
            return;
        case ZEND_AST_VAR:
        case ZEND_AST_DIM:
        case ZEND_AST_PROP:
        case ZEND_AST_STATIC_PROP:
        case ZEND_AST_CALL:
        case ZEND_AST_METHOD_CALL:
        case ZEND_AST_STATIC_CALL:
            zend_compile_var(result, ast, BP_VAR_R);
            return;
        case ZEND_AST_ASSIGN:
            zend_compile_assign(result, ast);
            return;
        case ZEND_AST_ASSIGN_REF:
            zend_compile_assign_ref(result, ast);
            return;
        case ZEND_AST_NEW:
            zend_compile_new(result, ast);
            return;
        case ZEND_AST_CLONE:
            zend_compile_clone(result, ast);
            return;
        case ZEND_AST_ASSIGN_OP:
            zend_compile_compound_assign(result, ast);
            return;
        case ZEND_AST_BINARY_OP:
            zend_compile_binary_op(result, ast);
            return;
        case ZEND_AST_GREATER:
        case ZEND_AST_GREATER_EQUAL:
            zend_compile_greater(result, ast);
            return;
        case ZEND_AST_UNARY_OP:
            zend_compile_unary_op(result, ast);
            return;
        case ZEND_AST_UNARY_PLUS:
        case ZEND_AST_UNARY_MINUS:
            zend_compile_unary_pm(result, ast);
            return;
        case ZEND_AST_AND:
        case ZEND_AST_OR:
            zend_compile_short_circuiting(result, ast);
            return;
        case ZEND_AST_POST_INC:
        case ZEND_AST_POST_DEC:
            zend_compile_post_incdec(result, ast);
            return;
        case ZEND_AST_PRE_INC:
        case ZEND_AST_PRE_DEC:
            zend_compile_pre_incdec(result, ast);
            return;
        case ZEND_AST_CAST:
            zend_compile_cast(result, ast);
            return;
        case ZEND_AST_CONDITIONAL:
            zend_compile_conditional(result, ast);
            return;
        case ZEND_AST_COALESCE:
            zend_compile_coalesce(result, ast);
            return;
        case ZEND_AST_PRINT:
            zend_compile_print(result, ast);
            return;
        case ZEND_AST_EXIT:
            zend_compile_exit(result, ast);
            return;
        case ZEND_AST_YIELD:
            zend_compile_yield(result, ast);
            return;
        case ZEND_AST_YIELD_FROM:
            zend_compile_yield_from(result, ast);
            return;
        case ZEND_AST_INSTANCEOF:
            zend_compile_instanceof(result, ast);
            return;
        case ZEND_AST_INCLUDE_OR_EVAL:
            zend_compile_include_or_eval(result, ast);
            return;
        case ZEND_AST_ISSET:
        case ZEND_AST_EMPTY:
            zend_compile_isset_or_empty(result, ast);
            return;
        case ZEND_AST_SILENCE:
            zend_compile_silence(result, ast);
            return;
        case ZEND_AST_SHELL_EXEC:
            zend_compile_shell_exec(result, ast);
            return;
        case ZEND_AST_ARRAY:
            zend_compile_array(result, ast);
            return;
        case ZEND_AST_CONST:
            zend_compile_const(result, ast);
            return;
        case ZEND_AST_CLASS_CONST:
            zend_compile_class_const(result, ast);
            return;
        case ZEND_AST_ENCAPS_LIST:
            zend_compile_encaps_list(result, ast);
            return;
        case ZEND_AST_MAGIC_CONST:
            zend_compile_magic_const(result, ast);
            return;
        case ZEND_AST_CLOSURE:
            zend_compile_func_decl(result, ast);
            return;
        default:
            ZEND_ASSERT(0 /* not supported */);
    }
}
/* }}} */

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6.3 zend_compile_echo 实现

# Zend/zend_compile.c
void zend_compile_echo(zend_ast *ast) /* {{{ */
{
    zend_op *opline;
    zend_ast *expr_ast = ast->child[0];

    znode expr_node;
    zend_compile_expr(&expr_node, expr_ast);

    opline = zend_emit_op(NULL, ZEND_ECHO, &expr_node, NULL);
    opline->extended_value = 0;
}

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