This is a function to create thumbnail image(png, jpeg, jpg or gif) with specific height...is useful when we need to make tables with the same height for each row with image previews...if you really need the same width and whatever height, just change the order of params in the aspect ratio operations (lines 17 and 18)...

create_thumbnail_function.php

public function createThumbnail($source_folder, $thumbs_folder, $source_file, $extension, $thumbHeight){
  	if ($extension == 'gif') {
			$imgt = "ImageGIF";
			$imgcreatefrom = "ImageCreateFromGIF";
		}else if($extension == 'jpg' || $extension == 'jpeg'){
			$imgt = "ImageJPEG";
			$imgcreatefrom = "ImageCreateFromJPEG";
		}else if ($extension == 'png') {
			$imgt = "ImagePNG";
			$imgcreatefrom = "ImageCreateFromPNG";
		}
		if ($imgt) {
			$img = $imgcreatefrom( $source_folder.$source_file.'.'.$extension );
			$width = imagesx( $img );
			$height = imagesy( $img );
     // keep aspect ratio with these operations...
			$new_width = floor( $width * ( $thumbHeight / $height ) );
			$new_height = $thumbHeight;
			$tmp_img = imagecreatetruecolor( $new_width, $new_height );
			if($extension == 'png'){
				// Disable alpha mixing and set alpha flag if is a png file
				imagealphablending($tmp_img, false);
				imagesavealpha($tmp_img, true);
			}
			imagecopyresized( $tmp_img, $img, 0, 0, 0, 0, $new_width, $new_height, $width, $height );
			$imgt( $tmp_img, $thumbs_folder.($source_file.'_'.$new_width.'x'.$new_height.'.'.$extension));
		}
	}