PHP array argument by value, gets modified when looping the array items by reference

Question

I have been having issues with an array being modified when passed by value to a function.

I have inspected the code and inside the function the array is looped getting the elements by reference.

I was surprised to see that after the loop the array items are marked as referenced. I don't know what this means, but must be the origin of my problem.

Let me put an example to see the point.

<?php
error_reporting(E_ALL);
ini_set('display_errors' , 1);

$a = array( array(0) );
echo '--1--';var_dump($a);
dummy($a);
echo '--4--';var_dump($a);

function dummy($arg) {
    foreach($arg as &$item) {
        $item[0] = 3;
    }    
    dummy2($arg);
echo '--3--';var_dump($arg);
}

function dummy2($arg) {
    foreach($arg as &$item) {
        $item[1]=9;
    }
echo '--2--';var_dump($arg);
}
?>

After this code I would expect that in point 3, $arg would have only one element, but it has two, so it has been modified by dummy2 function.

The output is as follows:

--1--array(1) { [0]=> array(1) { [0]=> int(0) } } 
--2--array(1) { [0]=> &array(2) { [0]=> int(3) [1]=> int(9) } } 
--3--array(1) { [0]=> &array(2) { [0]=> int(3) [1]=> int(9) } } 
--4--array(1) { [0]=> array(1) { [0]=> int(0) } }

Why are the arrays marked as &array after being looped by reference? How can this be avoid?

newacct · Accepted Answer · 2012-06-13 04:42:26Z

2

You need to unset the loop variable that captures by reference:

<?php
error_reporting(E_ALL);
ini_set('display_errors' , 1);

$a = array( array(0) );
echo '--1--';var_dump($a);
dummy($a);
echo '--4--';var_dump($a);

function dummy($arg) {
    foreach($arg as &$item) {
        $item[0] = 3;
    }
    unset($item);
    dummy2($arg);
echo '--3--';var_dump($arg);
}

function dummy2($arg) {
    foreach($arg as &$item) {
        $item[1]=9;
    }
    unset($item);
echo '--2--';var_dump($arg);
}
?>

See in the documentation for foreach, there is a big red warning that says:

Reference of a $value and the last array element remain even after the foreach loop. It is recommended to destroy it by unset().

answered Jun 13, 2012 at 4:42

newacct

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Steve Robbins · Accepted Answer · 2012-06-12 21:41:37Z

0

Use key => value pairs and return the array in your functions

<?php

    error_reporting(E_ALL);
    ini_set('display_errors' , 1);

    $a = array( array(0) );

    echo '--1--';var_dump($a);

    $a = dummy($a);

    echo '--4--';var_dump($a);

    function dummy($arg) {

        foreach($arg as $key => $value) {

            $arg[$key][0] = 3;
        }

        return dummy2($arg);
    }

    function dummy2($arg) {

        foreach($arg as $key => $value) {

           $arg[$key][1]=9;
        }

        return $arg;
    }
?>

http://codepad.org/f30c6FUj

answered Jun 12, 2012 at 21:41

Steve Robbins

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