preg_match() expects parameter 2 to be string on line 417 of wp-includes/class-wp-block-parser.php

[thenerdcave]

wp version 5.3.2
os MacOS 10.15.2
browser FireFox 71.0

Building a custom theme and on a very simple page if I have a qry param in the url that is an array (ex. ?c[]=3&c[]=6) I get the following warnings. If I change the url param to a string like c=3 then the warnings go away.

Warning: preg_match() expects parameter 2 to be string, array given in /wp-includes/class-wp-block-parser.php on line 417

Warning: strlen() expects parameter 1 to be string, array given in /wp-includes/class-wp-block-parser.php on line 489

The reason the url has these params is for a custom data filter with a multi-select menu.

After a google search it appears that this bug was introduced in version 5.3.1 and based on my findings to fix the issue that is accurate.

The bug appears to be related to the parse_blocks() function, I was able to trace the issue back to this location: blocks.php line 271 function filter_block_content();

If I update the function like so,

function filter_block_content( $text, $allowed_html = 'post', $allowed_protocols = array() ) {
	$result = '';

	if ( !is_array( $text ) ) {
		$blocks = parse_blocks( $text );
		foreach ( $blocks as $block ) {
			$block  = filter_block_kses( $block, $allowed_html, $allowed_protocols );
			$result .= serialize_block( $block );
		}
	}

	return $result;
}

by adding the if !is_array() condition the page works properly and warnings go away. I'm not 100% sure if this is the correct place for this fix but wanted to let you know about the issue, since I don't believe WP should be trying to parse qry params this way.

Hope this information is helpful.

[philippmuenchen]

On WP 5.4 this also breaks the Customizer when saving WP-Customizer fields that save arrays to theme_mod instead of strings. Example:

On "publish" in Customizer, wp-ajax.php is called with the parameter

&customized=%7B%22global_typo_copy%22%3A%7B%22font-family%22%3A%22Lato%22%2C%22font-weight%22%3A400%7D%2C%22global_typo_copy%5Bfont-family%5D%22%3A%22Lato%22%7D

The saving fails with

preg_match() expects parameter 2 to be string, array given at /Volumes/Webprojekte/sueco.de/web/wp/wp-includes/class-wp-block-parser.php:417

and the Customizer breaks without updating the value.

@thenerdcave suggestion fixes it for me, too.