Which means generating a string as per make_word takes more memory than it should.

sprintf-mem-hungry-strings.php

<?php

/**
 * 
 * When using sprintf and mt_rand to generate a random hex-like string, the
 * output seems to take more memory than it should.
 * 
 * Changing the output to a string via a (string) cast or by any other mean,
 * such as concatenating a string to it seems to fix the memory usage.
 * 
 * My output
 * string '      No trick: 15.51 M' (length=23)
 * string '    With trick: 4.45 M' (length=22)
 * string '         Delta: 11.06 M' (length=23)
 * string '       % delta: 71.31' (length=21)
 * string 'trick/no-trick: 0.29' (length=20)
 * 
 * @author Andre Tannus
 * 
 * $ php -v
 * PHP 5.4.4-14+deb7u2 (cli) (built: Jun  6 2013 03:36:44) 
 * Zend Engine v2.4.0, Copyright (c) 1998-2012 Zend Technologies
 *     with Xdebug v2.2.1, Copyright (c) 2002-2012, by Derick Rethans
 */


function make_word()
{
	$word  = sprintf('%04x', mt_rand(0, 0xffff));
	return $word;
}

function main()
{
	## Setup
	
	$max = 50000;
	
	## Measurements -----------------------------------------------------------
	 
	# No Trick
	$j = 0;
	$array_no_trick = array();
	$mem_no_trick_before = memory_get_usage();
	while( $j++ < $max )
	{
		$array_no_trick[] = make_word();
	}
	$mem_no_trick_after = memory_get_usage();

	# With Trick
	$j = 0;
	$array_with_trick = array();
	$mem_with_trick_before = memory_get_usage();
	while( $j++ < $max )
	{
		$array_with_trick[] = (string) make_word();
	}
	$mem_with_trick_after = memory_get_usage();
	

	## Maths ------------------------------------------------------------------
	
	$mb = 1024*1024;
	$mem_no_trick_delta = round( ($mem_no_trick_after-$mem_no_trick_before)/$mb, 2);
	$mem_with_trick_delta = round( ($mem_with_trick_after-$mem_with_trick_before)/$mb, 2);
	$delta_memory = $mem_no_trick_delta - $mem_with_trick_delta;
	$delta_percent = round( 100 * ($delta_memory / $mem_no_trick_delta),2 );

	
	## Display ----------------------------------------------------------------
	
	var_dump('      No trick: ' . $mem_no_trick_delta . ' M');
	var_dump('    With trick: ' . $mem_with_trick_delta . ' M');
	var_dump('         Delta: ' . $delta_memory . ' M');
	var_dump('       % delta: ' . $delta_percent);
	var_dump('trick/no-trick: ' . round($mem_with_trick_delta/$mem_no_trick_delta, 2));

}

main();
exit();