No matter what the original width and height is of the source image, this function will resize your source file to a particular width and height of your choosing. The function preserves the original aspect ratio and then just adds solid bars on the top or bottom, if needed.

ResizeImage.php

<!-- 
	Description:  No matter what the original width and height is of the source image, this function will resize your source file to a particular width and height of your choosing.  The function preserves the original aspect ratio and then just adds solid bars on the top or bottom, if needed.
	Author:  Joe Steinbring (http://steinbring.net)
	Date:  09/12/2014
-->

<?php 

function imageResize($NewWidth, $NewHeight, $NewFilename, $OldFilename) {
	// Get the details on the target $OldFilename
	$ImageDetails = getimagesize($OldFilename);
	// What's the mimetype?
	$MimeType = $ImageDetails["mime"];
	// What are the current dimensions?
	$CurrentWidth = $ImageDetails[0];
	$CurrentHeight = $ImageDetails[1];
	// Calculate the scale to fit the image inside of the target dimensions
	$scale = min($NewWidth/$CurrentWidth, $NewHeight/$CurrentHeight);
	// What should the actual new dimensions be, based upon the calculated scale?
	$CalcNewWidth = ceil($scale*$CurrentWidth);
	$CalcNewHeight = ceil($scale*$CurrentHeight);

	// Create new empty, new image
	$NewImage = imagecreatetruecolor($NewWidth, $NewHeight);

	// START: Make the background transparent
	imagesavealpha($NewImage, true);
	imagealphablending($NewImage, false);
	$white = imagecolorallocatealpha($NewImage, 255, 255, 255, 127);
	imagefill($NewImage, 0, 0, $white);
	// END: Make the background transparent

	// Read the file behind $OldFilename into variable
	if($MimeType == 'image/jpeg'){
		// It's a JPEG
		$OldImage = imagecreatefromjpeg($OldFilename);
	}else if($MimeType == 'image/gif'){
		// It's a GIF
		$OldImage = imagecreatefromgif($OldFilename);
	}else if($MimeType == 'image/png'){
		// It's a PNG
		$OldImage = imagecreatefrompng($OldFilename);
	}
	// Resize old image into new
	imagecopyresampled($NewImage, $OldImage, 
    	($NewWidth-$CalcNewWidth)/2, ($NewHeight-$CalcNewHeight)/2, 0, 0, 
    	$CalcNewWidth, $CalcNewHeight, $CurrentWidth, $CurrentHeight);

	// Write the image to the file behind $NewFilename
	if($MimeType == 'image/jpeg'){
		// It's a JPEG
		imagejpeg($NewImage, $NewFilename);
	}else if($MimeType == 'image/gif'){
		// It's a GIF
		imagegif($NewImage, $NewFilename);
	}else if($MimeType == 'image/png'){
		// It's a PNG
		imagepng($NewImage, $NewFilename);
	}

	// Clean up your stuff
	imagedestroy($NewImage);
	imagedestroy($OldImage);
}

imageResize(400, 300, 'BackDoor_med.jpg', 'BackDoor.jpg');

imageResize(150, 160, 'BackDoor_thmb.jpg', 'BackDoor.jpg');

imageResize(400, 300, 'MyBad_med.gif', 'MyBad.gif');

imageResize(150, 160, 'MyBad_thmb.gif', 'MyBad.gif');

imageResize(400, 300, 'coffee_med.png', 'coffee.png');

imageResize(150, 160, 'coffee_thmb.png', 'coffee.png');

?>

<!-- 
	Medium Image: 	400x300
	Thumbnail: 	150x160
-->

<img src="BackDoor.jpg">
<img src="BackDoor_med.jpg">
<img src="BackDoor_thmb.jpg">

<hr>

<img src="MyBad.gif">
<img src="MyBad_med.gif">
<img src="MyBad_thmb.gif">

<hr>

<img src="coffee.png">
<img src="coffee_med.png">
<img src="coffee_thmb.png">