Fix uninitialized variable wp_filter

[mikaelmayer]

In the file wp-settings.php, there is this code:

	// Re-initialize any hooks added manually by advanced-cache.php
	if ( $wp_filter ) {
		$wp_filter = WP_Hook::build_preinitialized_hooks( $wp_filter );
	}

However, $wp_filter is never set on a standard WordPress installation.
Therefore, a isset should be added, so that it does not trigger useless NOTICEs debug statements:

	// Re-initialize any hooks added manually by advanced-cache.php
	if ( isset($wp_filter) ) {
		$wp_filter = WP_Hook::build_preinitialized_hooks( $wp_filter );
	}

See the PR:
​https://github.com/WordPress/wordpress-develop/compare/master...MikaelMayer:patch-1

The variable is not set on a first installation of WordPress, so it is triggering a NOTICE debug error.
Although NOTICE errors are suppressed, this code smell should be fixed.

Trac ticket: https://core.trac.wordpress.org/ticket/51728#ticket

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I was not able to reproduce the error, probably something got corrupted in my installation.

[SergeyBiryukov]

Hi there, welcome to WordPress Trac! Thanks for the ticket.

Just to clarify, $wp_filter is declared as a global earlier in wp-includes/plugin.php​, so it is always set at this point and an isset() check would be redundant.

[mikaelmayer]

Oh I see, that was not on the same level. You're right. I'm testing a new VM for PHP and it looks like it was not taking these global definitions into account.