src/Debug/Utilities.php
public static function isList($val)
    {
        if (!\is_array($val)) {
            return false;
        }
        $keys = \array_keys($val);
        foreach ($keys as $i => $key) {
            if ($i != $key) {
                return false;
            }
        }
        return true;
    }
public static function isList($val)
    {
        if (!\is_array($val)) {
            return false;
        }
        $keys = \array_keys($val);
        foreach ($keys as $i => $key) {
            if ($i != $key) {
                return false;
            }
        }
        return true;
    }
Is passed argument a simple array with all-integer in sequence from 0 to n?
empty array returns true

@param [mixed $val value to check

@return boolean