Mgccl's Ivory Towerso tired

<?php
    $cool = imagecreatefrompng('imagefile.png');
    echo memory_get_usage(),'<br />';
 
      $timeparts = explode(' ',microtime());
  $starttime = $timeparts[1].substr($timeparts[0],1);
	  $cool = imagecreatefrompng('primespiral2000.png');
    //imagedestroy($cool);
    unset($cool);
      $timeparts = explode(' ',microtime());
  $endtime = $timeparts[1].substr($timeparts[0],1);
  echo bcsub($endtime,$starttime,6),'<br />';
    echo memory_get_usage(),'<br />';
    ?>

    $cool = imagecreatefrompng('imagefile.png');
    imagedestroy($cool);
echo isset($cool); //1
 $cool = imagecreatefrompng('imagefile.png');
    unset($cool);
echo isset($cool); //echos nothing...