PHP7 kernel analysis 4 local variables, global variables, constants

The content of this article is about local variables, global variables, and constants in PHP7 Kernel Analysis 4. Now I share it with you. Friends in need can refer to it

1. Local variables

Local variables in PHP are allocated on the zend_execute_data structure. Each time zend_op_array is executed, a new zend_execute_data will be generated. Local variables are allocated at the beginning of execution and then released at the end of execution. This is the local variable life cycle.

Read and write operations: Local variables are read and written through the number determined at compile time

2. Static variables

The storage location of static variables: zend_op_array->static_variables, this is a hash table, so static variables in PHP are different from ordinary local variables. They are not allocated in the execution space zend_execute_data, but in the hash table. The form is saved in zend_op_array. Static variables will only be initialized once. Note: Its initialization occurs in the compilation phase rather than the execution phase. This feature also means that the initial value of a static variable cannot be a variable, for example: static $count = $xxx; Such a definition will result in an error

Read and write operations: First, retrieve the corresponding zval from static_variables based on the variable name, then modify it to a reference type and assign it to a local variable. That is to say, static $count = 4; contains two operations, strictly $count is not a real static variable, it is just a local variable pointing to a static variable. The actual operation when executing is: $count = & static_variables["count"];

3. Global variables

Global variables always exist during the entire request execution, and they are saved in EG( executor_globals , Zend executor-related global variables) in the symbol_table, like the storage of static variables, this is also a hash table. Before zend_execute_ex execution starts, all global variables in the current scope will be added to the EG's symbol_table.

Read and write operations: Like the access of static variables, global variables also convert the original value into a reference, and then create a local variable in the scope of the global import to point to the reference

4. Constants

Constants are stored in EG’s zend_constants hash table and are also accessed Search directly in the hash table according to the constant name

typedef struct _zend_constant {
    zval value;   //常量值
    zend_string *name; //常量名
} zend_constant;

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