“Non-static method … cannot be called statically” on PHP 8: what it means and the fix

The message, on PHP 7.4 and PHP 8

A price helper formats amounts with a method called as Price_Tools::format(), a habit from older PHP code. The method isn’t declared static:

<?php
class Price_Tools {
    public function format($amount) {
        return '$' . number_format($amount, 2);
    }
}

echo Price_Tools::format(19.9), "\n";

Deprecated: Non-static method Price_Tools::format() should not be called statically in /var/www/html/wp-content/plugins/price-tools/price-tools.php on line 8
$19.90

Fatal error: Uncaught Error: Non-static method Price_Tools::format() cannot be called statically in /var/www/html/wp-content/plugins/price-tools/price-tools.php:8
Stack trace:
#0 {main}
  thrown in /var/www/html/wp-content/plugins/price-tools/price-tools.php on line 8

PHP 7.4 runs it and adds a deprecation notice. PHP 8.0 stops at the call. php.net, in the PHP 8.0 changes: “The ability to call non-static methods statically has been removed.” (php.net)

What it means

A class method is either static, called on the class itself (Price_Tools::format()), or non-static, called on an object made from the class ($tools->format()). Old PHP versions let code call a non-static method the static way, as long as the method didn’t use $this. PHP 8 doesn’t: the method must be declared static to be called on the class.

Through a WordPress hook: a different wording

Plugins often register a method on a hook as a text, add_action( 'init', 'My_Plugin::setup' ). If that method isn’t static, PHP 8 stops when WordPress runs the hook, with a longer message, because WordPress calls the method through call_user_func_array():

<?php
class My_Plugin {
    public function setup() {
        echo "Setup done\n";
    }
}

// The plugin registered 'My_Plugin::setup' on a hook; when the hook runs,
// WordPress calls it like this (wp-includes/class-wp-hook.php):
call_user_func_array('My_Plugin::setup', []);

Deprecated: Non-static method My_Plugin::setup() should not be called statically in /var/www/html/wp-content/plugins/my-plugin/my-plugin.php on line 10
Setup done

Fatal error: Uncaught TypeError: call_user_func_array(): Argument #1 ($callback) must be a valid callback, non-static method My_Plugin::setup() cannot be called statically in /var/www/html/wp-content/plugins/my-plugin/my-plugin.php:10
Stack trace:
#0 {main}
  thrown in /var/www/html/wp-content/plugins/my-plugin/my-plugin.php on line 10

On a real site, this message points to wp-includes/class-wp-hook.php, the file where WordPress runs hooks. WordPress isn’t the cause: the method named in the message, My_Plugin::setup(), tells you which plugin is.

Why you didn’t see it coming

PHP 7.4 did warn, with a deprecation notice. But while WP_DEBUG is off, WordPress doesn’t report deprecation notices (WordPress 7.1.2 source), so on a normal live site the warning was never written anywhere.

Fix it, in order

Find it before you switch

CompatNav doesn’t check for this pattern: our scan of the example above found nothing. Your PHP 7.4 site can tell you, though: set WP_DEBUG and WP_DEBUG_LOG to true, with WP_DEBUG_DISPLAY false (the WordPress debug log), use the site for a while, and search the log for “should not be called statically”. Each line is a fatal error on PHP 8. Other PHP 8.0 changes that stop old code: create_function(), each(), old-style constructors, all of which CompatNav does find.