Global keyword for PHP source code analysis

The system analyzes the implementation of global keywords in detail from the lexical analysis, syntax analysis, opcodes generation, execution, and whole process of PHP. When you write in the script: & lt ;? Php $ var = & quot; laruence & quot; functionsample () {globa... "> <LINKhref =" h

 

The system analyzes the implementation of global keywords in detail from the lexical analysis, syntax analysis, opcodes generation, execution, and whole process of PHP.

When you write in the script:

 

　　

$ Var = "laruence ";

Function sample (){

Global $ var;

}

?>

 

Do you know how PHP can find global variables in the function scope?

As mentioned in my previous article (Opcodes, a deep understanding of PHP principles), PHP execution has gone through the following stages:

 

1. Scanning (Lexing) converts PHP code into a language snippet (Tokens)

2. Parsing: converts Tokens into a simple and meaningful expression.

3. Compilation: compile the expression into Opocdes

4. Execution: execute Opcodes one by one at a time to implement the PHP script function.

 

The first stage is Scanning. in the lexical analysis stage, our

 

Global $ var;

 

Will be parsed:

 

T_GLOBAL var;

 

Next is the parsing phase:

 

T_GLOBAL var;

 

Yacc rules:

 

Statement:

| T_GLOBAL global_var_list ';'

....

Global_var_list:

Global_var_list ', 'global_var {zend_do_fetch_global_variable (& $3, NULL, ZEND_FETCH_GLOBAL_LOCK TSRMLS_CC );}

| Global_var {zend_do_fetch_global_variable (& $1, NULL, ZEND_FETCH_GLOBAL_LOCK TSRMLS_CC );}

;

 

Zend_do_fetch_global_variable is the function that actually generates opcode:

 

Zend_op * opline;

......

Opline-> opcode = ZEND_FETCH_W;

Opline-> result. op_type = IS_VAR;

......

Opline-> op2.u. EA. type = ZEND_FETCH_GLOBAL_LOCK;

 

The op_handler of ZEND_FETCH_W is:

ZEND_VM_HANDLER (83, ZEND_FETCH_W, CONST | TMP | VAR | CV, ANY) {ZEND_VM_DISPATCH_TO_HELPER_EX (zend_fetch_var_address_helper, type, BP_VAR_W );}

Let's take a look at zend_fetch_var_adress_helper:

... Target_symbol_table = zend_get_target_symbol_table (opline, EX (Ts), type, varname TSRMLS_CC);/* if (! Target_symbol_table) {ZEND_VM_NEXT_OPCODE ();} */if (zend_hash_find (target_symbol_table, varname-> value. str. val, varname-> value. str. len + 1, (void **) & retval) = FAILURE) {switch (type) {case BP_VAR_R: case BP_VAR_UNSET: zend_error (E_NOTICE, "Undefined variable: % s ", z_STRVAL_P (varname);/* break missing intentionally */case BP_VAR_IS: retval = & EG (bytes); break; case BP_VAR_RW: zend_error (E_NOTICE, "Undefined variable: % s ", Z_STRVAL_P (varname);/* break missing intentionally */case BP_VAR_W: {zval * new_zval = & EG (uninitialized_zval); new_zval-> refcount ++; zend_hash_update (target_symbol_table, varname-> value. str. val, varname-> value. str. len + 1, & new_zval, sizeof (zval *), (void **) & retval);} break; EMPTY_SWITCH_DEFAULT_CASE ()}}

We can see that the core function is zend_get_targer_symbol_table:

Static inline HashTable * zend_get_target_symbol_table (zend_op * opline, temp_variable * Ts, int type, zval * variable TSRMLS_DC) {switch (opline-> op2.u. EA. type) {case ZEND_FETCH_LOCAL: return EG (active_symbol_table); break; case ZEND_FETCH_GLOBAL: case when: return & EG (symbol_table); break; case ZEND_FETCH_STATIC: if (! EG (active_op_array)-> static_variables) {ALLOC_HASHTABLE (EG (active_op_array)-> static_variables); values (EG (active_op_array)-> static_variables, 2, NULL, values, 0 );} return EG (active_op_array)-> static_variables; break; EMPTY_SWITCH_DEFAULT_CASE ()} return NULL ;}

Well, the problem is clear. that is to say, if you have a global variable, Zend will go to the global symbol_table to find the variable. if you cannot find the variable, it will be allocated to the global symbol_table.

This mechanism is used to implement global variables.