[dcphp-dev] Memory Usage in PHP Arrays

Colleagues:  I'd like some help understanding what is going on here.  Here is a little script that you can copy and run to see the outputs that are annotated in the comments.  Questions follow.

<?php // RAY_temp_julianh.php
error_reporting(E_ALL);
echo '<pre>';

// URL OF THE INPUT FILE
$url = 'http://www.laprbass.com/RAY_temp_julianh.csv';  // FILESIZE IS 10,788,344 BYTES

// CHECK THE MEMORY USAGE
$use = memory_get_usage(TRUE);
var_dump($use);                  // int(786432)

// READ THE FILE INTO A STRING
$str = file_get_contents($url);
$use = memory_get_usage(TRUE);
var_dump($use);                  // int(11796480)

// MAKE AN ARRAY
$arr = explode(PHP_EOL, $str);
$use = memory_get_usage(TRUE);
var_dump($use);                  // int(221773824)

// DELETE THE STRING VARIABLE
unset($str);
$use = memory_get_usage(TRUE);
var_dump($use);                  // int(221773824)

// DELETE THE ARRAY VARIABLE
unset($arr);
$use = memory_get_usage(TRUE);
var_dump($use);                  // int(12058624)

The output of the first var_dump() makes sense, and given the file size, the output of the second var_dump() after reading the file makes sense.  Then I am confused.

The output from the third var_dump() after creating the array seems to indicate that array creation took a 10MB string and multiplied its memory size by nearly a factor of 20.  This caused me to do an immediate WTF, so I tested it by making an object with a property for each element.  Very similar numbers.  Why does it take 20X the storage to represent an array or object?

The fourth var_dump() after unset() of the 10MB input string showed no change in memory usage.  Why was no memory released?  Or is memory_get_usage(TRUE) reporting something that is understood in the art but is opaque to me? For example, is there a minimum memory allocation?

The fifth var_dump() after unset() of the array seems to have released memory associated with the array, but leaving only 256K unaccounted for.  Why?

Thanks for any insight you can share.  Best to all, Ray

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