Mine

function setMemoryForImage( $filename ){
   $imageInfo = getimagesize($filename);
   $MB = 1048576;
   $K64 = 65536;
   $TWEAKFACTOR = 1.5;
   $memoryNeeded = round( ( $imageInfo[0] * $imageInfo[1]
                                           * $imageInfo['bits']
                                           * $imageInfo['channels'] / 8
                             + $K64
                           ) * $TWEAKFACTOR
                         );
   //ini_get('memory_limit') only works if compiled with "--enable-memory-limit" also
   $memoryLimit = 8;
   if (function_exists('memory_get_usage') &&
       memory_get_usage() + $memoryNeeded > $memoryLimit)
   {
       $newLimit = $memoryLimit + ceil( ( memory_get_usage()
                                           + $memoryNeeded
                                           - $memoryLimit
                                           ) / $MB
                                       );
       ini_set( 'memory_limit', $newLimit . 'M' );
       return true;
   }else
       return false;
}