Are strings referenced in PHP5?

Question

Are strings referenced or copied, when passed as arguments or assigned to variables, in PHP5?

Answer 1

The debug_zval_dump() function might help you answer this question.

For example, if I run the following portion of code :

$str = 'test';
debug_zval_dump($str);      // string(4) "test" refcount(2)

my_function($str);
debug_zval_dump($str);      // string(4) "test" refcount(2)

function my_function($a) {
    debug_zval_dump($a);    // string(4) "test" refcount(4)
    $plop = $a . 'glop';
    debug_zval_dump($a);    // string(4) "test" refcount(4)
    $a = 'boom';
    debug_zval_dump($a);    // string(4) "boom" refcount(2)
}

I get the following output :

string(4) "test" refcount(2)
string(4) "test" refcount(4)
string(4) "test" refcount(4)
string(4) "boom" refcount(2)
string(4) "test" refcount(2)

So, I would say :

• Strings are "refcounted", when passed to functions (and, probably, when assigned to variables)
• BUT do not forget that PHP does copy on write

For more informations, here are a couple of links that might be useful :

• Reference Counting Basics
• Do not use PHP references
• Maitrise de la gestion des variables en PHP (in french)

Answer 2

They are copies or dereferenced.