Retrieve appropriate image size in WordPress (eg. when moving from TimThumb to regular WP sizes).

wp-retrieve-appropriate-size.php

<?php
/**
 * @author Fabian Wolf
 * @link http://usability-idealist.de/
 * @license GNU GPL v2 or later
 */
// try fetching an appropriate size setting based on width, optionally on height, too
		if( !empty( $width ) ) {
			// fetch available sizes
			$arrImageSizes = $this->get_image_sizes();
			
			// retrieve anything with the width <= current width
			foreach( $arrImageSizes as $strSize => $arrSizeAtts ) {
				if( $arrSizeAtts['width'] <= $width ) {
					$arrPossibleSizes[ $strSize ] = $arrSizeAtts;
					$arrWidths[ $strSize ] = $arrSizeAtts[ 'width'];
					
					if( !empty( $height ) && $arrSizeAtts['height'] > $height ) {
						unset( $arrPossibleSizes[ $strSize ] );
						unset( $arrWidths[ $strSize ] );
					}
						
				}
			}
			
			if( !empty( $arrPossibleSizes ) ) {
				// sort widths
				natsort( $arrWidths );
				
				$arrRWidths = array_reverse( $arrWidths, true );
				$c = 0;
				
				$strAppropiateSize = '';
				foreach( $arrRWidths as $strSize => $iWidth ) {
					if( $c == 0 ) {
						$strAppropiateSize = $strSize;
						break;
					}
					
					$c++;
				}
				// eg. width = 200, height = 150 .. 
				
				if( !empty( $strAppropiateSize ) ) {
					$atts['size'] = $strAppropiateSize;
				}
			}
		}
		

NOTE: This is just a snippet out of some actual work I'm doing right now. Didnt want it to go under though. Its supposed to be used in a backward compatiblity hack for the WP gallery shortcode (gallery_shortcode()).