How are variables passed in PHP?

Variables in PHP are passed by default by value. This means that when assigning the value of one variable to another or passing a variable to a function, a copy of the value is created. Changes to this copy do not affect the original variable.

However, variables can also be passed by reference. In this case, instead of a copy of the value, a pointer to the same memory area as the original variable is passed. Changes made through the reference directly affect the original variable.

Passing by reference is used:

• Assignment by reference: Using the assignment operator with an ampersand (=&).

  // Assignment by reference
  $a = 10;
  $b =& $a;
  $b = 20; // Changing $b also changes $a
  // $a now equals 20
  

• Passing to a function by reference: Adding an ampersand (&) before the parameter name in the function definition.

  <?php
  // Function accepting an argument by reference
  function modifyValue(&$value) {
      $value *= 2;
  }
  
  $number = 5;
  modifyValue($number);
  // $number now equals 10
  ?>
  

• Returning a value by reference from a function: Using an ampersand (&) before the function name in its definition.

  <?php
  // Function returning a value by reference
  function &getReference() {
      static $value = 0;
      $value++;
      return $value;
  }
  
  $a = &getReference();
  $a = 100; // Changing $a modifies the static variable inside the function
  
  $b = &getReference(); // $b now references the same static variable (equal to 101)
  ?>
  

It is important to note that some data types, such as objects, are always passed by reference (more accurately, a reference to the object is passed). When assigning one object to another, both variables refer to the same object instance.

Method of passing	How it works	Application
By value	A copy of the value is created	Default behavior, safe for the original variable
By reference	A pointer to the memory is passed	Changes directly affect the original variable