[wp-trac] [WordPress Trac] #63751: wpdb->set_prefix() throws deprecation notice for preg_match() when setup-config.php is loaded

[WordPress Trac]

#63751: wpdb->set_prefix() throws deprecation notice for preg_match() when setup-
config.php is loaded
-------------------------------------------------+-------------------------
 Reporter:  jerclarke                            |      Owner:  (none)
     Type:  defect (bug)                         |     Status:  new
 Priority:  normal                               |  Milestone:  Awaiting
                                                 |  Review
Component:  Database                             |    Version:  6.8.2
 Severity:  minor                                |   Keywords:
  Focuses:  coding-standards, php-compatibility  |
-------------------------------------------------+-------------------------
 A bit esoteric, but this is throwing a reliable deprecation notice in my
 logs when bots visit `/wp-admin/setup-config.php`:

 {{{
 [25-Jul-2025 18:59:23] PHP deprecated: "preg_match(): Passing null to
 parameter #2 ($subject) of type string is deprecated" file: /[...]/wp-
 includes/class-wpdb.php:1007 url: [...]/wp-admin/setup-config.php
 }}}

 The code throwing the error:

 {{{
 public function set_prefix( $prefix, $set_table_names = true ) {

         if ( preg_match( '|[^a-z0-9_]|i', $prefix ) ) {
                 return new WP_Error( 'invalid_db_prefix', 'Invalid
 database prefix' );
         }
 }}}


 The immediate cause is obvious: `preg_match()` is run without checking
 that `$prefix` has any content, and in PHP 8.x you're not allowed to pass
 empty strings to it.

 The "real" cause, I don't know. Why is there no `$prefix` available when
 normally there would be? Should there be?

 These `WP_INSTALLING` files like `setup-config.php` are a mystery to me,
 and seem relatively untested especially with these PHP 8.x edge-cases.

 My goal with this ticket is just to clean up the deprecation warning.

 It seems to me all that needs to be done is to check that `!empty(
 $prefix)` before doing the preg_replace(), so:

 {{{
 public function set_prefix( $prefix, $set_table_names = true ) {

         if ($prefix && preg_match( '|[^a-z0-9_]|i', $prefix ) ) {
                 return new WP_Error( 'invalid_db_prefix', 'Invalid
 database prefix' );
         }
 }}}

 Seems to me that would resolve the notice and have the exact same effect
 as the old code.

 Thank you for helping fix this 🙏🏻

-- 
Ticket URL: <https://core.trac.wordpress.org/ticket/63751>
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