Why Reference to Reference Not Working Inside Foreach Loop?-CodePudding

In below code if I uncomment(use) Approach 2 & comment(disable) Approach 1, why Approach 2 is not modifying the $arr? What is are the alternatives to Approach 2?

$val = 'OOOOOO';
$arr = ['a' => 'AAA', 'b' => 'BBB'];

echo print_r($arr, true) . '<br>'; //Array ( [a] => AAA [b] => BBB )


// Approach 1 start - THIS WORKS
$arr['a'] = &$val;
$arr['b'] = &$val;
// Approach 1 end

// Approach 2 start - DOES NOT WORKS
// foreach ($arr as $ky => &$vl) {
//     $vl = &$val;
// }
// Approach 2 end

echo print_r($arr, true) . '<br>'; //Array ( [a] => OOOOOO [b] => OOOOOO )

CodePudding user response：

In the foreach loop you can't modifiy your $vl without a reference. Since it's not the original variable.

In PHP (>= 5.0), is passing by reference faster?

What you can do is something like this:

foreach ($array as $key => $value) { 
  /* your condition*/
  $array[$key] = $some_value;
}

Therefore in the foreach you can use $value (that can be modified without consequences) but you can also modify directly your array. Modifications that would be kept outside of the foreach

CodePudding user response：

Found solution for me: Approach 3

Approach 3 (pass by reference foreach loop) is faster than Approach 4 (pass by value foreach loop) (same as Gregoire Ducharme answer):

// Approach 3 start
foreach ($arr as $ky => &$vl) {
    $arr[$ky] = &$val;
}
// Approach 3 end

// Approach 4 start
foreach ($arr as $ky => $vl) {
    $arr[$ky] = &$val;
}
// Approach 4 end

Performance Data (tested on array with long text, 1000000 loops):

By Reference (Approach 3):

Time: 0.32825803756714, Memory Peak Usage: 596672

Time: 0.33496713638306, Memory Peak Usage: 596672

Time: 0.33242797851562, Memory Peak Usage: 596672

Time: 0.35326600074768, Memory Peak Usage: 596672

Time: 0.33251810073853, Memory Peak Usage: 596672

By Value (Approach 4):

Time: 0.36558699607849, Memory Peak Usage: 596672

Time: 0.38936495780945, Memory Peak Usage: 596672

Time: 0.42887806892395, Memory Peak Usage: 596672

Time: 0.44330382347107, Memory Peak Usage: 596672

Time: 0.49368786811829, Memory Peak Usage: 596672