Return a WordPress image thumbnail without image dimensions

get_post_thumbnail_without_dimensions.php

/**
 * Get an HTML img element representing an image attachment
 * Based on wp_get_attachment_image(), without height and width.
 *
 * While $size will accept an array, it is better to register a size with
 * add_image_size() so that a cropped version is generated. It's much more
 * efficient than having to find the closest-sized image and then having the
 * browser scale down the image.
 *
 * @since 2.0
 * @see add_image_size()
 * @uses apply_filters() Calls 'wp_get_attachment_image_attributes' hook on attributes array
 * @uses wp_get_attachment_image_src() Gets attachment file URL and dimensions
 * @since 2.5.0
 * @package Rocket Lift Parent Theme
 *
 * @param int $attachment_id Optional, defaults to null, which will use current post image ID. Image attachment ID.
 * @param string $size Optional, default is 'thumbnail'.
 * @param bool $icon Optional, default is false. Whether it is an icon.
 * @param array $attr Optional, default is empty. Attributes to pass (e.g. src, class, alt, title).
 * @return string HTML img element or empty string on failure.
 */
function get_post_thumbnail_without_dimensions($attachment_id = '', $size = 'thumbnail', $icon = false, $attr = '') {

    global $post;
    
    if ( '' == $attachment_id ) {
        if ( has_post_thumbnail( $post->ID ) ) {
            $attachment_id = get_post_thumbnail_id( $post->ID );
        } else {
            return false;
        }
    }

    $html = '';
    $image = wp_get_attachment_image_src($attachment_id, $size, $icon);
    if ( $image ) {
        list($src, $width, $height) = $image;
        if ( is_array($size) )
            $size = join('x', $size);
        $attachment =& get_post($attachment_id);
        $default_attr = array(
            'src'   => $src,
            'class' => "attachment-$size",
            'alt'   => trim(strip_tags( get_post_meta($attachment_id, '_wp_attachment_image_alt', true) )), // Use Alt field first
            'title' => trim(strip_tags( $attachment->post_title )),
        );
        if ( empty($default_attr['alt']) )
            $default_attr['alt'] = trim(strip_tags( $attachment->post_excerpt )); // If not, Use the Caption
        if ( empty($default_attr['alt']) )
            $default_attr['alt'] = trim(strip_tags( $attachment->post_title )); // Finally, use the title

        $attr = wp_parse_args($attr, $default_attr);
        $attr = apply_filters( 'wp_get_attachment_image_attributes', $attr, $attachment );
        $attr = array_map( 'esc_attr', $attr );
        $html = rtrim("<img ");
        foreach ( $attr as $name => $value ) {
            $html .= " $name=" . '"' . $value . '"';
        }
        $html .= ' />';
    }

    return $html;
}