strict-typing.php

<?php 

declare(strict_types=1);

/**
 * @param bool $error
 * @param null $message
 * @return string
 */
function printError(bool $error, $message = null): ?string
{
    if ($error) {
        $message = 'You have encountered an error';
        // Initial null variable now contains a string value
        // even with strict type enforcement
    }

    return $message;
}

$message = printError(true, null);

echo $message; // 'You have encountered an error'