PHP is a simple and powerful language that provides many language features suitable for the Web, including weak typing of variables. Under the weak typing mechanism, you can assign any type of value to a variable.
PHP is executed through Zend Engine (hereinafter referred to as ZE). ZE is written in C and implements a set of weak type mechanisms at the bottom. ZE's memory management uses optimization strategies such as copy-on-write and reference counting to reduce memory copies when reassigning variables.
The following not only takes you to explore the principles of PHP weak typing, but also writes about PHP extensions and introduces how to operate PHP variables.
1. PHP variable types
There are 8 variable types in PHP:
PHP does not strictly check the variable type. Variables can declare their type without displaying it, and assign values directly during runtime. Variables can also be converted freely. As in the following example, without implementation declaration, $i can be assigned any type of value.
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If you don’t have a deep understanding of the principle of weak typing, you will have “exceeding expectations” surprises when comparing variables.
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All the above three results They are equal because PHP performs variable conversion internally when comparing variables. If you want the value and type to be determined at the same time, please use three = (for example, $a===0) to determine. Maybe you will find it commonplace, maybe you will find it amazing, then please join me to delve into the PHP kernel and explore the principle of PHP variables.
2. Introduction to variable storage and standard types
All variables in PHP are implemented with the structure zval. In Zend/zend.h we can see the definition of zval:
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Among them, refcount__gc and is_ref__gc indicate whether the variable is a reference. The type field identifies the type of the variable. The value of type can be: IS_NULL, IS_BOOL, IS_LONG, IS_FLOAT, IS_STRING, IS_ARRAY, IS_OBJECT, IS_RESOURCE. PHP chooses how to store zvalue_value based on the type. [php] view plaincopy
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5. Conversion of variable types Follow Now what we know about the PHP language, variables The type depends on the zval.type field indication, and the content of the variable is stored in zval.value according to zval.type. When variables are needed in PHP, only two steps are required: change the value or pointer of zval.value, and then change the type of zval.type. However, for some of PHP's advanced variables Array/Object/Resource, variable conversion requires more operations. Variable conversion principles are divided into 3 types: 5.1 Standard type mutual conversion is relatively simple, just follow the above steps for conversion. 5.2 Standard type and resource type conversion The resource type can be understood as int, which is more convenient for converting standard types. After conversion, the resource will be closed or recycled. [php] view plaincopy
5.3 Standard type and complex type conversion Array conversion int/floating point type Float will return the number of elements; conversion to bool will return whether there are elements in Array; conversion to string will return 'Array' and throw a warning. 5.4 Complex type mutual conversion array and object can be converted to each other. If any other type of value is converted to an object, an instance of the built-in class stdClass will be created. When we write PHP extensions, the PHP kernel provides a set of functions for type conversion:
A set of macros provided by the PHP kernel to conveniently access zval and obtain the value of zval in a more fine-grained manner:
6. Variable symbol table and scope PHP’s variable symbol table and zval value mapping is through HashTable (hash table, also called hash table, hereinafter referred to as HT). HashTable is widely used in ZE, including Language features such as constants, variables, and functions are organized by HT, and the array type in PHP is also implemented through HashTable. [php] view plaincopy The variable name of $var will be stored in the variable symbol table and represents $ The zval structure of the var's type and value is stored in a hash table. The kernel implements access to PHP variables through the hash mapping of the variable symbol table and the zval address. Why do we need to mention scope? Because the internal variables of the function are protected. According to the scope, PHP variables are divided into global variables and local variables. Each scope PHP maintains a HashTable of symbol tables. When creating a function or class in PHP, ZE will create a new symbol table to indicate that the variables in the function or class are local variables. This achieves the protection of local variables - variables inside the function cannot be accessed from the outside. When creating a PHP variable, ZE will assign a zval, set the corresponding type and initial value, and add the variable to the symbol table of the current scope so that the user can use the variable.
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When writing a PHP extension, you can access PHP's variable symbol table through the EG macro. EG (symbol_table) accesses the variable symbol table of the global scope, and EG (active_symbol_table) accesses the variable symbol table of the current scope. The local variable stores a pointer, which is passed to the corresponding function when operating on the HashTable. In order to better understand the hash table and scope of variables, let’s take a simple example:
$temp =
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