Calling wp_parse_args() early, with $args as empty string, may cause fatal error

[johnjamesjacoby]

When running the unit test suite while working on #31245, I ran into an edge-case fatal error:

Fatal error: Uncaught Error: Call to undefined function wp_parse_str() in /srv/www/wordpress-develop/public_html/src/wp-includes/functions.php on line 3487

Error: Call to undefined function wp_parse_str() in /srv/www/wordpress-develop/public_html/src/wp-includes/functions.php on line 3487

Anytime between functions.php being loaded and formatting.php being loaded, calling wp_parse_args() with an empty string as the first parameter will trigger it, and I was doing this in the wp_load_alloptions() stack with my test code.

It happens because wp_parse_args() calls wp_parse_str() when $args is an empty string, and because formatting.php is loaded well after functions.php, the above fatal happens.

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It's not a bug in core today, but it could be a problem later.

In my research, one place of relatively high risk is register_setting(). You'd need to pass an empty string as the third parameter, and it would need to be early in the stack, but it calls wp_parse_args() and is in functions.php.

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One potential solution is to move wp_parse_str() out of formatting.php and into functions.php, alongside it's other wp_parse_ siblings. Another would be to load formatting.php sooner.

[johnjamesjacoby]

I've found 28 places in core where $args = '' is used as a parameter definition. It's theoretically possible calling those functions without any parameter variables and before formatting.php is included could also trigger the above fatal error.

This PR addresses a potential fatal error in the wp_parse_args() function, which can occur if wp_parse_args() is called with an empty string as the first parameter before wp_parse_str() is defined. The issue arises because wp_parse_str(), which is required by wp_parse_args(), is located in formatting.php, which is loaded later in the WordPress load order than functions.php. This results in an undefined function fatal error when wp_parse_args() calls wp_parse_str() before formatting.php has been loaded.

Trac ticket: https://core.trac.wordpress.org/ticket/40552