is_numeric

<?php             $operator = $version[0];

            $version = substr($version, 1);


        } elseif (is_numeric($version[0])) {

            $operator = '==';

        } else {

            throw new InvalidArgumentException('The version must start with [<, >, <=, >=] or a number.');

<?php expect()->intercept('toBe', fn ($value, $expected) => $value === '*' && is_numeric($expected), function ($expected) use ($state) {

    $state->runCount['wildcard']++;

    $state->appliedCount['wildcard']++;

});