Creating thumbnail with fixed dimensions

[.Stealth]

What's the best way of doing this?

 

I want to make thumbnails of an image, but have a fixed thumbnail size. My current function does what i want, just the new image only has a small cut out of the original with the rest of the image black even though the original image is bigger than the thumbnail.

 

Current function:

 

function createImage($srcImage, $extension, $newWidth, $newHeight, $saveTo){
$oldWidth	= imageSX($srcImage);//get the width of the source image
$oldHeight	= imageSY($srcImage);//get the height of the source image

$oldWidth	= $oldWidth - $newWidth;
$oldHeight 	= $oldHeight - $newHeight; 	

$newImageX	= $oldWidth/2;
$newImageY 	= $oldHeight/2;

$dstImg = ImageCreateTrueColor($newWidth,$newHeight);
imagecopyresampled($dstImg,$srcImage,0,0,$newImageX,$newImageY,$newWidth,$newHeight,$oldWidth,$oldHeight);

if($extension == 'jpg'){//if the file is a jpeg
	imagejpeg($dstImg, $saveTo, 100);
}
if($extension == 'png'){//if the file is a png
	imagepng($dstImg, $saveTo);
}
imagedestroy($srcImage);
}

 

any advice? Thanks.