How to fix "Illegal string offset 'output_key'" in nav_menu.php

Question

I've got a website on Wordpress and someone in my team updated something and now there is an error on that website:

Warning: Illegal string offset 'output_key' in /.../wp-includes/nav-menu.php

I disabled the debug message and now the site is working fine. I guess this is not the best solution. Hours of googling and looking for something here brought me to the fact that I have to tell the script that 'output_key' is an array element.

$defaults = array( 'order' => 'ASC', 'orderby' => 'menu_order', 'post_type' => 'nav_menu_item',
        'post_status' => 'publish', 'output' => ARRAY_A, 'output_key' => 'menu_order', 'nopaging' => true );
    $args = wp_parse_args( $args, $defaults );
    $args['include'] = $items;


if ( ARRAY_A == $args['output']) {
        $GLOBALS['_menu_item_sort_prop'] = $args['output_key'];
        usort($items, '_sort_nav_menu_items');
        $i = 1;
        foreach( $items as $k => $item ) {
            $items[$k]->$args['output_key'] = $i++; //here is the error
        }
    }

I tried to ask $args if it is an array in line 1.

$defaults = array( 'order' => 'ASC', 'orderby' => 'menu_order', 'post_type' => 'nav_menu_item',
        'post_status' => 'publish', 'output' => ARRAY_A, 'output_key' => 'menu_order', 'nopaging' => true );
    $args = wp_parse_args( $args, $defaults );
    $args['include'] = $items;


if ( ARRAY_A == $args['output'] && is_array($args) ) {
        $GLOBALS['_menu_item_sort_prop'] = $args['output_key'];
        usort($items, '_sort_nav_menu_items');
        $i = 1;
        foreach( $items as $k => $item ) {
            $items[$k]->$args['output_key'] = $i++; //here is the error
        }
    }

But line 6 still shows an error on the website. I don't know what to do with that line.

Answer 1

It seems that you are using an outdated version of wordpress which is no longer compatible with PHP 7.

See migrating from php 5.6 to php 7 section about variable handling.

Which means that the expression $items[$k]->$args['output_key'] is interpreted as:

• $items[$k]->{$args['output_key']} in PHP 5
• ($items[$k]->$args)['output_key'] in PHP 7

It also seems that this issue is fixed in recent wordpress code.

To fix the issue manually, just replace the code:

$items[$k]->$args['output_key'] = $i++; // here is the error

with:

$items[$k]->{$args['output_key']} = $i++; // problem solved :)

You should also consider upgrading entire wordpress installation to make sure that the rest of the code is compatible with PHP7.