Resize images dynamically using WordPress' built in image manipulation functions.

vt_resize.php

<?php
/**
 * Resize images dynamically using wp built in functions.
 * @author Victor Teixeira http://profiles.wordpress.org/vteixeira/
 * @author Aaron Collegeman http://aaroncollegeman.com
 * @see http://core.trac.wordpress.org/ticket/15311
 *
 * php 5.2+
 *
 * Examples:
 *
 * <?php 
 *   $image = vt_resize( get_post_thumbnail_id(), 140, 110, true ); // == vt_resize( get_the_ID(), 140, 110, true )
 *   // or, resize any locally stored image:
 *   $image = vt_resize( '/some/image/url.jpg', 100, 100, true );
 * ?>
 * <img src="<?php echo $image[url]; ?>" width="<?php echo $image[width]; ?>" height="<?php echo $image[height]; ?>" />
 *
 * @param mixed $attach_ref - either an attachment id, a post ID, or a URL
 * @param int $width
 * @param int $height
 * @param bool $crop
 * @return array
 */
function get_vt_resize( $attach_ref = null, $width, $height, $crop = false ) {

  // this is an attachment or a Post, so we have the ID
  if ( is_numeric($attach_ref) ) {
  
    $image_src = wp_get_attachment_image_src( $attach_ref, 'full' );
    if (!$image_src) {
      $attach_ref = get_post_thumbnail_id( $attach_ref );
      $image_src = wp_get_attachment_image_src( $attach_ref, 'full' );
    }

    $file_path = get_attached_file( $attach_ref );
  
  // this is not an attachment, so try to find the file
  } else {
    
    $file_path = parse_url( $attach_ref );
    $file_path = ltrim( $file_path['path'], '/' );
    //$file_path = rtrim( ABSPATH, '/' ).$file_path['path'];
    
    $orig_size = getimagesize( $file_path );
    
    $image_src[0] = $attach_ref;
    $image_src[1] = $orig_size[0];
    $image_src[2] = $orig_size[1];
  }

  if (!$file_path) {
    return array(
      'url' => '#',
      'width' => -1,
      'height' => -1
    );
  }
  
  $file_info = pathinfo( $file_path );
  $extension = '.'. $file_info['extension'];

  // the image path without the extension
  $no_ext_path = $file_info['dirname'].'/'.$file_info['filename'];

  $cropped_img_path = $no_ext_path.'-'.$width.'x'.$height.$extension;

  // checking if the file size is larger than the target size
  // if it is smaller or the same size, stop right here and return
  if ( $image_src[1] > $width || $image_src[2] > $height ) {

    // the file is larger, check if the resized version already exists (for crop = true but will also work for crop = false if the sizes match)
    if ( file_exists( $cropped_img_path ) ) {

      $cropped_img_url = str_replace( basename( $image_src[0] ), basename( $cropped_img_path ), $image_src[0] );
      
      $vt_image = array (
        'url' => $cropped_img_url,
        'width' => $width,
        'height' => $height
      );
      
      return $vt_image;
    }

    // crop = false
    if ( $crop == false ) {
    
      // calculate the size proportionaly
      $proportional_size = wp_constrain_dimensions( $image_src[1], $image_src[2], $width, $height );
      $resized_img_path = $no_ext_path.'-'.$proportional_size[0].'x'.$proportional_size[1].$extension;      

      // checking if the file already exists
      if ( file_exists( $resized_img_path ) ) {
      
        $resized_img_url = str_replace( basename( $image_src[0] ), basename( $resized_img_path ), $image_src[0] );

        $vt_image = array (
          'url' => $resized_img_url,
          'width' => $new_img_size[0],
          'height' => $new_img_size[1]
        );
        
        return $vt_image;
      }
    }

    // no cached files - let's finally resize it
    $new_img_path = image_resize( $file_path, $width, $height, $crop );
    $new_img_size = getimagesize( $new_img_path );
    $new_img = str_replace( basename( $image_src[0] ), basename( $new_img_path ), $image_src[0] );

    // resized output
    $vt_image = array (
      'url' => $new_img,
      'width' => $new_img_size[0],
      'height' => $new_img_size[1]
    );
    
    return $vt_image;
  }

  // default output - without resizing
  $vt_image = array (
    'url' => $image_src[0],
    'width' => $image_src[1],
    'height' => $image_src[2]
  );
  
  return $vt_image;
}

/**
 * Use this function to simply print the URL of the resized image.
 * Note that this approach does not account for cases in which the
 * source image is smaller than the requested size, in which case
 * the requested width/height will not necessarily be the exact width
 * and height of the resulting image.
 *
 * Example:
 *
 * <img src="<?php vt_resize(get_the_ID(), 400, 300, true) ?>" width="400" height="300" />
 *
 * @param mixed $attach_ref - either an attachment id, a post ID, or a URL
 * @param int $width
 * @param int $height
 * @param bool $crop
 */
function vt_resize( $attach_ref = null, $width, $height, $crop = false ) {
  $image = get_vt_resize( $attach_ref, $width, $height, $crop );
  echo $image['url'];
}