Potential assignment to empty string

[jugglinmike]

Some of WordPress's built-in shortcodes assume that the provided $attr parameter
is an associative array. For instance, img_caption_shortcode is currently
defined with the following code:

    1489	/**
    1490	 * Builds the Caption shortcode output.
    [...]
    1503	 * @param array  $attr {
    1504	 *     Attributes of the caption shortcode.
    [...]
    1516	 */
    1517	function img_caption_shortcode( $attr, $content = null ) {
    1518	        // New-style shortcode with the caption inside the shortcode with the link and image tags.
    1519	        if ( ! isset( $attr['caption'] ) ) {
    1520	                if ( preg_match( '#((?:<a [^>]+>\s*)?<img [^>]+>(?:\s*</a>)?)(.*)#is', $content, $matches ) ) {
    1521	                        $content         = $matches[1];
    1522	                        $attr['caption'] = trim( $matches[2] );
    1523	                }
    1524	        } elseif ( strpos( $attr['caption'], '<' ) !== false ) {
    1525	                $attr['caption'] = wp_kses( $attr['caption'], 'post' );
    1526	        }

However, the shortcode parser can potentially return the empty string:

    483	/**
    484	 * Retrieve all attributes from the shortcodes tag.
    [...]
    493	 * @return array|string List of attribute values.
    494	 *                      Returns empty array if trim( $text ) == '""'.
    495	 *                      Returns empty string if trim( $text ) == ''.
    496	 *                      All other matches are checked for not empty().
    497	 */
    498	function shortcode_parse_atts( $text ) {

In PHP7, this behavior can cause problems for shortcodes which do not specify
any attributes. (Prior releases of PHP exposed ​surprising behavior for this case which averted the issue.)

To migrate my deployment to PHP7, I've preserved the behavior by overriding shortcodes on an as-needed basis:

    function img_caption_shortcode_supporting_empty( $attr, $content = null ) {
      if ( $attr == '' ) {
        $attr = array();
      }

      return img_caption_shortcode($attr, $content);
    }

    remove_shortcode('caption');
    add_shortcode('caption', 'img_caption_shortcode_supporting_empty');

But it seems like a few things should change in core:

• updating the documented type of the $attr parameter of shortcode functions
• tolerating the empty string

[jugglinmike]

Correction: ​the relevant change to PHP was in 7.1, not 7.0