Code Modernization: Fix null to non-nullable deprecation in WP_Term_Query::parse_orderby()

[itzmekhokan]

WP_Term_Query::parse_orderby() passes the raw orderby query var straight to strtolower(). When that value is null, PHP 8.1+ emits a deprecation notice. This casts the value to string first, which silences the notice without altering any existing behaviour.

What the problem was:

• get_terms( array( 'orderby' => null ) ) emitted strtolower(): Passing null to parameter #1 ($string) of type string is deprecated on PHP 8.1 and above, at wp-includes/class-wp-term-query.php:921.
• 'orderby' defaults to 'name', but wp_parse_args() lets a caller override it with an explicit null, so the value reaches strtolower() unguarded.
• The method already handles a falsy value correctly further down — the empty( $_orderby ) branch orders by t.term_id. Only the notice was wrong.

What the fix does:

• Casts the raw value to string before lowercasing: strtolower( (string) $orderby_raw ).
• Adds a regression test covering a null orderby.

Approach and why:

• One line, at the exact point of failure. This matches how Core has fixed the same class of issue in sibling query classes — see [51806], which guarded WP_Comment_Query::get_comment_ids() at its call site rather than reworking the method.
• A (string) cast was chosen over an is_string() guard (the idiom used by the neighbouring parse_order()) deliberately. The cast reproduces PHP's pre-8.1 implicit coercion exactly for every scalar type — null/false → '', 5 → '5', true → '1' — so behaviour is unchanged for all of them.
• An is_string() guard would not be behaviour-preserving: parse_orderby_meta() resolves the value via array_key_exists( $orderby_raw, $meta_clauses ), and unnamed meta_query clauses receive integer keys. PHP normalises numeric string keys to integers, so array_key_exists( '5', array( 5 => ... ) ) is true — meaning a numeric orderby can legitimately reference a meta clause today. Discarding non-strings would silently break that ordering.
• The @param string docblock is left as-is; the cast is defensive against callers, not a signature change.

Trac ticket: https://core.trac.wordpress.org/ticket/65679

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Pull request overview

This PR updates WP_Term_Query::parse_orderby() to avoid PHP 8.1+ deprecation notices caused by passing null to strtolower() when orderby is explicitly set to null, and adds a PHPUnit regression test for that scenario.

Changes:

• Casts the raw orderby value before lowercasing in WP_Term_Query::parse_orderby().
• Adds a term query test asserting that orderby => null falls back to ordering by term ID.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
tests/phpunit/tests/term/getTerms.php	Adds a regression test ensuring orderby => null orders by term ID.
src/wp-includes/class-wp-term-query.php	Adjusts parse_orderby() to prevent PHP 8.1+ null-to-string deprecation in strtolower().

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[Soean]

orderby should be a string, why should we allow null? What is the usecase?

https://developer.wordpress.org/reference/classes/wp_term_query/__construct/#parameters

[itzmekhokan]

orderby should be a string, why should we allow null? What is the usecase?

https://developer.wordpress.org/reference/classes/wp_term_query/__construct/#parameters

Fair question — you're right that nothing in core passes null here. Every core call site into WP_Term_Query passes a string literal, so this can only originate in plugin or theme code. There's no legitimate use case for null, and I don't think there should be one.

One things that made me think it's still worth hardening, the sibling query var is already hardened in this same class. $order is documented as @type string exactly like $orderby, yet parse_order() has guarded against non-strings since WP_Term_Query was introduced in [37572]:

if ( ! is_string( $order ) || empty( $order ) ) {
     return 'DESC';
}

So order is defended and orderby isn't — that asymmetry looks more like an oversight than a deliberate line.

It's really about where the failure surfaces — the notice names class-wp-term-query.php:921 with nothing pointing at the actual caller.

If you'd rather not absorb caller type errors here, I'm happy to close this and have the ticket resolved as invalid — your call.