In-depth understanding of PHP principles: variable scope (Scope in PHP)

The previous article (In-depth understanding of PHP principles: Variables inside PHP) introduced the internal representation of PHP variables, but the problem is, how do these internal representations interact with user scripts? What are the variables associated with? In other words, if I write in the script:

<?php
  $var = "laruence";
  echo $var;
?>

How does ZE connect my variable var to the internal structure zval?

As mentioned in the in-depth understanding of PHP principles and variables, PHP internally uses zval to represent variables, but for the above script, our variables have names, var. There is no corresponding field in zval to reflect the variable name.

If you think of it, there must be a mechanism inside PHP to map variable names to zval. Then you are really smart;)

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In PHP, all variables will be stored in an array (

hash table to be precise), and PHP also implements variables through different arrays of the scope.

When you create a variable, PHP will assign a zval to the variable, fill in the corresponding variable value, and then fill in the name of the variable and the pointer to the zval into an array. Then, when you get this variable, PHP will get the corresponding zval by looking up this array.

View the _zend_executor_globals structure (this structure saves some execution-related context information in the PHP executor)

struct _zend_executor_globals {
 
     ....
    HashTable *active_symbol_table;/*活动符号表*/
    HashTable symbol_table;     /*全局符号表*/
 
    HashTable included_files;
 
    jmp_buf *bailout;
    int error_reporting;
     .....
}

Among them,

Global symbol table , saves variables in the top-level scope (that is, not within any function or object). Whenever a function (method of an object) is called, an active symbol table will be created for this function, and all variables defined within this function will be saved in this active symbol table.

Yes, this is how PHP's variable scope is implemented! For example:

<?php
     $var = "I am in the global symbol table";
    function sample($para){
        $var = "I am in the active symbol table";
          echo $var;
      }
    sample($var);
    echo $var;
  ?>

The variable $var outside the function sample will be filled in the global symbol table, and it will be filled in with the global symbol table. It corresponds to a zval pointer, which saves a string "I am in the global symbol table".

The $var in the function will be filled in the active symbol table belonging to the function sample , the same, the string "I am in the active symbol table" is stored in the zval corresponding to it.

What is special is the parameter $para of the function sample. This $para is stored in sample's active symbol table, but the zval pointer corresponding to it will point to a zval that holds a copy of the global variable $var (strictly speaking, it is not a copy, but a reference).

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