Generating serie of numbers and letters

I want to generate a serie of numbers from 1 to 5, I wrote the following code

<?php
   $tag = range(1,5);
   echo "Numbers : ".implode(" - F",$tag)."<br>";
?>

I expect to get this result :

F1 - F2 - F3 - F4 - F5

But I am getting that instead :

1 - F2 - F3 - F4 - F5

What should I modify to get the wanted result ?