Detailed analysis of the underlying development principles of PHP7: the process from syntax analysis to semantic analysis

Detailed analysis of the underlying development principles of PHP7: the process from syntax analysis to semantic analysis

The underlying development principles of programming languages ​​are basic knowledge that programmers must master. In the underlying development of PHP7, syntax parsing and semantic analysis are two very important processes. This article will analyze these two processes in detail and attach corresponding code examples.

1. Grammar parsing
Grammar parsing is the process of converting PHP code into a syntax tree. Its main task is to convert codes in the form of strings into structured data that can be understood by computers. In PHP7, syntax parsing is implemented using generators and parsers.

1. Generator
   A generator is a function that can use the yield statement to return a value and retain the current state of the function. Generators are a flexible and efficient implementation that save memory and CPU usage. The following is a simple generator example:

function myGenerator() {
    yield 'Hello';
    yield 'World';
}

$gen = myGenerator();

foreach ($gen as $value) {
    echo $value . ' ';
}

// 输出结果：Hello World

In the above code, the myGenerator() function is a generator function that uses the yield statement to return two values ​​respectively. In a foreach loop, the generator function is executed and a value is returned each time through the loop.

2. Parser
   The parser is the key component that converts PHP code into a syntax tree. In PHP7, the parser is implemented using lexical analysis and syntax analysis.

• Lexical analysis: The lexical analyzer parses the source code into individual tags, such as variable names, keywords, operators, etc. The following is a simple lexical analysis example:

$code = '<?php $a = 1 + 2; ?>';

$tokens = token_get_all($code);

foreach($tokens as $token) {
    if (is_array($token)) {
        echo "Line {$token[2]}: {$token[1]}".PHP_EOL;
    }
}

// 输出结果：
// Line 1: <?php
// Line 1: $
// ...

In the above code, the token_get_all() function parses the source code into tokens and stores them in the tokens array. By looping through the tokens array, we can output the content and line number of each token.

• Grammar analysis: The grammar analyzer converts the tokens obtained by lexical analysis into a syntax tree. The following is a simple syntax analysis example:

$code = '<?php $a = 1 + 2; ?>';

$tokens = token_get_all($code);

$parser = new Parser();

$ast = $parser->parse($tokens);

print_r($ast);

// 输出结果：
// Array
// (
//     [0] => Array
//         (
//             [name] => AST_ASSIGN
//             [left] => Array
//                 (
//                     [name] => AST_VAR
//                     [value] => $a
//                      ...

In the above code, the token_get_all() function parses the source code into tokens and stores them in the tokens array. We then use the parse() method of the Parser class to convert the token into a syntax tree, which will be returned as an array.

2. Semantic analysis
Semantic analysis is the process of analyzing and processing syntax trees. It is mainly used to check code consistency and logical errors, and perform type inference. In PHP7, semantic analysis is implemented using symbol tables and type systems.

1. Symbol table
   The symbol table is a data structure used to store information about variables and functions in the code. In semantic analysis, the symbol table plays an important role and can check problems such as variable redefinition and variable type errors. The following is a simple symbol table example:

$symbol_table = array();

function addVariable($name, $type) {
    global $symbol_table;
    
    if (isset($symbol_table[$name])) {
        throw new Exception("Variable {$name} is already defined");
    }
    
    $symbol_table[$name] = $type;
}

function getVariableType($name) {
    global $symbol_table;
    
    if (!isset($symbol_table[$name])) {
        throw new Exception("Variable {$name} is not defined"); 
    }
    
    return $symbol_table[$name];
}

addVariable('a', 'int');
addVariable('b', 'string');

echo getVariableType('a'); // 输出结果：int
echo getVariableType('b'); // 输出结果：string

In the above code, we use a global variable $symbol_table to store variable information. The addVariable() function is used to add variables to the symbol table, and the getVariableType() function is used to obtain the type of the variable. If the variable is already defined, an exception is thrown.

2. Type system
   The type system is a mechanism for type checking variables and expressions. In semantic analysis, the type system can check for problems such as type mismatches and type conversion errors. The following is a simple type system example:

function add($a, $b) {
    return $a + $b;
}

echo add(1, 2); // 输出结果：3
echo add("Hello", "World"); // 输出结果：HelloWorld
echo add(1, "World"); // 抛出异常：Invalid operand types

In the above code, the add() function is used to add two operands. When the operand type is an integer, the add() function performs an addition operation; when the operand type is a string, the add() function performs a string concatenation operation. If the operand types do not match, an exception is thrown.

Summary:
The underlying development principles of PHP7 involve two core processes: syntax parsing and semantic analysis. Syntax parsing is the process of converting PHP code into a syntax tree, which is implemented using generators and parsers. Semantic analysis is the process of analyzing and processing syntax trees, which is implemented using symbol tables and type systems. A deep understanding of these two processes is very important for underlying development and code optimization.

Through the detailed analysis and code examples of this article, I believe that readers will have a deeper understanding of the underlying development principles of PHP7 and be able to apply them to actual development. I hope this article can help you, and I wish you go further and further on the road of underlying development of PHP7!

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