Question

I have an array of size 5 MB, I am passing it to functions to functions (I am not passing it by reference)

foo( $arr );

function foo( $arr ) {
    ....
    bar( $arr );
    ....
}

function bar( $arr ) {
    ....
    test( $arr );
    ....
}

function test( $arr ) {
    ....
    test2( $arr );
    ....
}

PHP passes array values by value (copy of value) by default to functions.

my question is, if this array value is passed to 100 functions calls, will the PHP consumes 100 x 5 MB = 500 MB of memory ?

How the PHP handles big arrays ( in memory wise ) on function calls?

Answer 1

Here is a code to test:

<?php

    function testnochanges($arr1){
        foreach($arr1 as $val){
            //
        }
        return $arr1;
    }

    function testwithchanges($arr1){
        $arr1[] = 1;
        return $arr1;
    }

    echo "Stage 0: Mem usage is: " . memory_get_usage() . "<br />";

      for ($i = 0; $i < 100000; ++$i) {
        $arr[] = rand();
      }

     echo "Stage 1 (Array Created): Mem usage is: " . memory_get_usage() . "<br />";


     $arrtest1 = testnochanges($arr);
     echo "Stage 2 (Function did NO changes to array): Mem usage is: " . memory_get_usage() . "<br />";

     $arrtest2 = testwithchanges($arr);
     echo "Stage 3 (Function DID changes to array): Mem usage is: " . memory_get_usage() . "<br />";


    ?>

and here is a output after i run it:

Stage 0: Mem usage is: 330656
Stage 1 (Array Created): Mem usage is: 8855296
Stage 2 (Function did NO changes to array): Mem usage is: 8855352
Stage 3 (Function DID changes to array): Mem usage is: 14179864

On stage 0 we can see that before array is created PHP is already using some space in memory. After creating first array (Stage 1) we can see a big change in memory usage as expected. But after calling function testnochanges function and creating $arrtest1 on Stage 2, we see that memory usage did not change a lot. It’s because we did no changes to $arr, so $arrtest1 and $arr still are pointing to the same array. But on Stage 3, where we call testwithchanges function, and add an element to $arr PHP performs copy-on-write and returned array which is assigned to $arrtest2 now uses different part of memory and again we see a big grow of memory usage.

Dry conclusion: If you copy array to another array and do not change it, memory usage stays the same as both arrays are pointed to the same one. If you change the array PHP performs copy-on-write and, of course memory usage grows.

Good thing to read: Be wary of garbage collection, part 2.