Creating thumbnail images, used to work, now doesnt work

[jwhite68]

My existing code has the following function, which uses some form of 'convert' function to create a thumbnail file.  However, its not working on the web server - even though it used to.  I have inherited this code, and do not know the background behind it.

 

Does anyone recognise it, or know what this 'convert' function is?  See 'this line' marker below.

I cannot see any PHP function called convert, and assume its some library that needs to be compiled into PHP, or maybe referencing a PERL function?

 

function resizeByWidth($input,$output,$width,$onlyIfBigger=true){
/*
Description : Resizes regular image by given height
Parameters :
	input :  path to the file
	output :path to output file 
	height :the new height
	onlyIfBigge : wheather to resize if image height is smaller then new height 		
Returns : 
	true : if success 
	false : if file not exists or not a regular image
*/
if($pic_info = checkImage($input)){
	if($pic_info[1]<$width && $onlyIfBigger){
		copy($input,$output);
		echo'File copy';
		return false;
	}
	$proportions = $width/$pic_info[1];
	$newHeight = round($pic_info[2] * $proportions); 
	$cmd = 'convert '.$input.' -thumbnail '.$width.'x'.$newHeight.' '.$output;  <--- this line
	$res = `$cmd`;
	echo $res;
}
else{
	return false;
}
}