1.2.917. PHP 7.4 Reserved Keyword

fn is a new PHP keyword. In PHP 7.4, it is used to build the arrow functions. When used at an illegal position, fn generates a Fatal error at compile time.

As a key word, fn is not allowed as constant name, function name, class name or inside namespaces. fn is fine for method names. It may also be used for constants with define(), and constant() but it is not recommended.

<?php

// PHP 7.4 usage of fn
function array_values_from_keys($arr, $keys) {
    return array_map(fn($x) => $arr[$x], $keys);
}

// PHP 7.3 usage of fn
const fn = 1;

function fn() {}

class x {
    // This is valid in PHP 7.3 and 7.4
    function fn() {}
}

?>

See also PHP RFC: Arrow Functions.

1.2.917.1. Specs

Short name	Php/Php74ReservedKeyword
Rulesets	All, CE, CompatibilityPHP74
Exakat since	1.9.2
PHP Version	With PHP 7.4 and older
Severity	Minor
Time To Fix	Quick (30 mins)
Precision	Very high
Available in	Entreprise Edition, Community Edition, Exakat Cloud