Ticket #38929: test-wp-hook.php

<?php

function append_1($a) { return $a . 1; }
function append_2($a) { return $a . 2; }
function append_3($a) { return $a . 3; }

// Before WP_Hook loads, we'll define a filter that should automatically convert.
$wp_filter['test_filter'][10]['test-key'] = array( 'function' => 'append_1', 'accepted_args' => 1 );

include 'src/wp-includes/plugin.php';

// This will output `01`, as it's been migrated to a WP_Hook
echo 'filter1: ' . apply_filters( 'test_filter', 0 ) . "\n";

// This will apply fine
add_filter( 'test_filter', 'append_2' );
// and output '012'
echo 'filter1: ' . apply_filters( 'test_filter', 0 ) . "\n";

// This will fail, with a `PHP Notice: Indirect modification of overloaded element of WP_Hook has no effect`
$wp_filter['test_filter'][10]['test-key-3'] = array( 'function' => 'append_3', 'accepted_args' => 1 );
// This will echo 012 again.
echo 'filter1: ' . apply_filters( 'test_filter', 0 ) . "\n";
// However, this will succeed (Thanks to the `WP_Hook::offsetSet()` method
$wp_filter['test_filter'][10] = array( 'test-key-3' => array( 'function' => 'append_3', 'accepted_args' => 1 ) );
// But will have overwritten the exisitng filters and echo `03`
echo 'filter1: ' . apply_filters( 'test_filter', 0 ) . "\n";

// Direct modifications to append something would have to be done like so:
$ten = $wp_filter['test_filter'][10];
$ten['test-key-2'] = array( 'function' => 'append_2', 'accepted_args' => 1 );
$wp_filter['test_filter'][10] = $ten;

// and will now echo `032`.
echo 'filter1: ' . apply_filters( 'test_filter', 0 ) . "\n";

// -----

// This filter will be added just-fine though, as it's not already set
$wp_filter['test_filter2'][10]['test-key-2'] = array( 'function' => 'append_1', 'accepted_args' => 1 );

// This would `Fatal error: Uncaught Error: Call to a member function apply_filters() on array`
// echo 'filter2: ' . apply_filters( 'test_filter2', 0 ) . "\n";

// Because the filters hadn't been 'upgraded' to WP_Hook after the `$wp_filter` modifications.
$wp_filter = WP_Hook::build_preinitialized_hooks( $wp_filter ); 
// This now outputs `01`.
echo 'filter2: ' . apply_filters( 'test_filter2', 0 ) . "\n";

// Filter1 will still output `032` though because it was preserved.
echo 'filter1: ' . apply_filters( 'test_filter', 0 ) . "\n";

echo "\n";