Using an array containing a single object as first $arg in do_action()

[pskli]

When passing an array containing 1 single object as the first argument of do_action(), the array is automatically flattened and all functions triggered by the hook will receive a single object as the first argument (instead of an array).

Example:

$var = [
        (object) [
                'key'   => 'name',
                'value' => 'Pierre',
        ]
]; 

do_action( 'hook_name', $var );

Any function hooked to hook_name receives this first argument:

stdClass Object
(
    [key] => name
    [value] => Pierre
)

…instead of an array containing this object.

I've found that this is because of some PHP4 backward compatibility in the do_action() function:

} elseif ( is_array( $arg[0] ) && 1 === count( $arg[0] ) && isset( $arg[0][0] ) && is_object( $arg[0][0] ) ) {
        // Backward compatibility for PHP4-style passing of `array( &$this )` as action `$arg`.
        $arg[0] = $arg[0][0];
}

This is a weird and unexpected behavior, could we add an additional condition in this elseif in order to check for the PHP version to apply this hack?

https://core.trac.wordpress.org/ticket/55133

Removing the PHP4 backward-compatibility in the do_action() function to avoid unexpected behaviour where the first argument is an array of a single object, as it's currently flattened and become a variable containing the single object.

Trac ticket: [](https://core.trac.wordpress.org/ticket/55133)

[audrasjb]

Hello @pskli and thanks for raising this,

We don't need PHP4 backwards compatibility anymore so removing this fallback makes sense, however I'm feeling a bit hesitant since changing this could break plugins relying on the current behavior.

This change would at least need a dev note, and probably further investigation to make sure this is not a massive breaking change in the ecosystem.

[azouamauriac]

introduced here: [46568]

[pskli]

Honestly, I'm wondering how this kind of behavior mentioned in this ticket has not been mentioned previously by plugin devs, as:

function do_action_with_array_of_unique_object() {
        $var = [ get_post( 1 ) ];
        do_action( 'some_action', $var );
}

function debug_unique_post_variable( $array_with_one_wp_post ) {
        error_log( print_r( $array_with_one_wp_post, true ) );
}
add_action( 'some_action', __NAMESPACE__ . '\\debug_unique_post_variable', 10, 1 );

…would result in getting a WP_Post instead of an array.

I agree that it could break plugins relying on the current behavior, but I believe that they're probably doing an ! is_array( $var ) { $var = [ $var ]; } to bypass this unexpected result, don't you think?

The current result, in this (rare?) specific case (where $arg should be an array containing 1 object) is unexpectedly weird.

[dlh]

Duplicate of #40284?

[SergeyBiryukov]

#55998 was marked as a duplicate.

[SergeyBiryukov]

#40284 was marked as a duplicate.