quack.php doesn't run

Copying this to a separate issue for searchability, quoting @Ax3l-91

https://github.com/aidantwoods/RPi0w-keyboard/issues/1#issuecomment-310905023

> Hi again @aidantwoods,
>
> I wanted to try your quack.php script but I have this error relative to this line:
> `function translate(string $l) : array
> PHP Parse error: syntax error, unexpected ':', expecting '{' in /home/pi/quack.php on line 47`
>
> Thanks in advance for your help!