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<?php

add_filter('image_downsize', function ($false, $attachment_id, $size)
{
    if (!wp_attachment_is_image($attachment_id))
        return FALSE;
    
    //---------------------------------------------------
    $upload_dir = wp_upload_dir();

    $size_string = is_array($size)? "{$size[0]}x{$size[1]}" : $size;
    
    $img_metadata = wp_get_attachment_metadata($attachment_id);
    

    // If indicated size not exist:
    if (!isset($img_metadata['sizes'][$size_string]))
    {
        // If $size is an Array, it will be used to create the new image.
        if (is_array($size))
        {
            $img_full_path = "{$upload_dir['basedir']}/{$img_metadata['file']}";
            
            $image = wp_get_image_editor($img_full_path);
 
            if (is_wp_error($image))
                return FALSE;
            
            if (isset($size[3]))
                $image->set_quality($size[3]);
            
            $crop = isset($size[2])? $size[2] : TRUE;

            $image->resize($size[0], $size[1], $crop);
            
            $result = $image->save();
            
            if (is_wp_error($result))
                return FALSE;
 

            $img_metadata['sizes'][$size_string] = array(
                'file' => $result['file'],
                'width' => $result['width'],
                'height' => $result['height'],
                'mime-type' => $result['mime-type']
            );
 
            wp_update_attachment_metadata($attachment_id, $img_metadata);
        }

        // Otherwise, return false.
        else
        {
            return FALSE;
        }
    }
 

    $img_full_url = "{$upload_dir['baseurl']}/{$img_metadata['file']}";
    $img_resized_url = str_replace(wp_basename($img_metadata['file']), $img_metadata['sizes'][$size_string]['file'], $img_full_url);
    $is_intermediate = TRUE;


    return array(
        $img_resized_url, 
        $img_metadata['sizes'][$size_string]['width'], 
        $img_metadata['sizes'][$size_string]['height'], 
        $is_intermediate
    );
}, 10, 3);

<?php

/**
 * Default:
 */
 
$post_thumbnail_id = get_post_thumbnail_id(get_the_ID());
$width = 640;
$height = 480;

$image_obj = wp_get_attachment_image_src($post_thumbnail_id, array($width, $height));

printf('<img src="%s" width="%u" height="%u">', $image_obj[0], $image_obj[1], $image_obj[2]);


/**
 * Other options:
 */
 
$post_thumbnail_id = get_post_thumbnail_id(get_the_ID());
$width = 640;
$height = 480;
$crop = true; // See WP_Image_Editor reference for cropping options.
$quality = 85;

$image_obj = wp_get_attachment_image_src($post_thumbnail_id, array($width, $height, $crop, $quality));

printf('<img src="%s" width="%u" height="%u">', $image_obj[0], $image_obj[1], $image_obj[2]);