PHP, Memory and Iteration - php

Question

I've been noticing some odd behavior while experimenting with benchmarking SplFixedArrays. Take this little snippet of code, for instance...
<?php
$splFixedArray = new \SplFixedArray( 100000 );
echo number_format( memory_get_usage() ) . PHP_EOL;
$variable = 'Truffaut single-origin coffee wayfarers, church-key asymmetrical 90\'s trust fund hashtag before they sold out thundercats photo booth. Godard sustainable roof party keffiyeh, Odd Future chillwave mlkshk kogi VHS leggings hoodie art party next level dreamcatcher yr. Blog american apparel aesthetic tattooed farm-to-table, stumptown viral whatever mixtape raw denim Williamsburg skateboard flexitarian actually tofu. Echo Park lomo disrupt PBR, jean shorts irony fingerstache blog kale chips. Street art iPhone PBR fingerstache Bushwick Cosby sweater. McSweeney\'s mumblecore semiotics, twee quinoa tofu +1 fingerstache pop-up. Echo Park bitters disrupt irony. Truffaut single-origin coffee wayfarers, church-key asymmetrical 90\'s trust fund hashtag before they sold out thundercats photo booth. Godard sustainable roof party keffiyeh, Odd Future chillwave mlkshk kogi VHS leggings hoodie art party next level dreamcatcher yr. Blog american apparel aesthetic tattooed farm-to-table, stumptown viral whatever mixtape raw denim Williamsburg skateboard flexitarian actually tofu. Echo Park lomo disrupt PBR, jean shorts irony fingerstache blog kale chips. Street art iPhone PBR fingerstache Bushwick Cosby sweater.';
var_dump( $variable );
for( $i = 0; $i < 100000; $i++ )
{
$splFixedArray[ $i ] = $variable;
}
echo number_format( memory_get_usage() );
Which outputs...
1,032,080
string(1209) "Truffaut single-origin coffee wayfarers, church-key asymmetrical 90's trust fund hashtag before they sold out thundercats photo booth. Godard sustainable roof party keffiyeh, Odd Future chillwave mlkshk kogi VHS leggings hoodie art party next level dreamcatcher yr. Blog american apparel aesthetic tattooed farm-to-table, stumptown viral whatever mixtape raw denim Williamsburg skateboard flexitarian actually tofu. Echo Park lomo disrupt PBR, jean shorts irony fingerstache blog kale chips. Street art iPhone PB"...
1,032,384
Now, let's add a simple random integer onto the end while in the for loop...
<?php
$splFixedArray = new \SplFixedArray( 100000 );
echo number_format( memory_get_usage() ) . PHP_EOL;
$variable = 'Truffaut single-origin coffee wayfarers, church-key asymmetrical 90\'s trust fund hashtag before they sold out thundercats photo booth. Godard sustainable roof party keffiyeh, Odd Future chillwave mlkshk kogi VHS leggings hoodie art party next level dreamcatcher yr. Blog american apparel aesthetic tattooed farm-to-table, stumptown viral whatever mixtape raw denim Williamsburg skateboard flexitarian actually tofu. Echo Park lomo disrupt PBR, jean shorts irony fingerstache blog kale chips. Street art iPhone PBR fingerstache Bushwick Cosby sweater. McSweeney\'s mumblecore semiotics, twee quinoa tofu +1 fingerstache pop-up. Echo Park bitters disrupt irony. Truffaut single-origin coffee wayfarers, church-key asymmetrical 90\'s trust fund hashtag before they sold out thundercats photo booth. Godard sustainable roof party keffiyeh, Odd Future chillwave mlkshk kogi VHS leggings hoodie art party next level dreamcatcher yr. Blog american apparel aesthetic tattooed farm-to-table, stumptown viral whatever mixtape raw denim Williamsburg skateboard flexitarian actually tofu. Echo Park lomo disrupt PBR, jean shorts irony fingerstache blog kale chips. Street art iPhone PBR fingerstache Bushwick Cosby sweater.';
var_dump( $variable );
for( $i = 0; $i < 100000; $i++ )
{
$splFixedArray[ $i ] = $variable . rand();
}
echo number_format( memory_get_usage() );
Which results in this...
1,034,320
string(1209) "Truffaut single-origin coffee wayfarers, church-key asymmetrical 90's trust fund hashtag before they sold out thundercats photo booth. Godard sustainable roof party keffiyeh, Odd Future chillwave mlkshk kogi VHS leggings hoodie art party next level dreamcatcher yr. Blog american apparel aesthetic tattooed farm-to-table, stumptown viral whatever mixtape raw denim Williamsburg skateboard flexitarian actually tofu. Echo Park lomo disrupt PBR, jean shorts irony fingerstache blog kale chips. Street art iPhone PB"...
129,834,272
What I'm curious about is why function calls are resulting in stacked memory usage. Is it normal that memory would not be freed up after the iteration?

Answer 1

This behavior is expected.
In the first case, you are storing the same value multiple times, which can be implemented as a single instance of the value and a batch of references to it, with a copy-on-write semantic for cases when the value at a given array index is changed.
In the second case, you are storing many different values, which can't be handled the same way; memory must be allocated for the full contents of each value, which results in the difference you see in memory consumption between the two cases.