As you are aware to generate a random number you can do something like this

rand(1,6);

This wont work for letters so youy cant do something like rand(a,z); . Instead we need to get the ascii value of our letter in the range .
To get the ascii value of a letter for example the letter "a" we do this

echo ord("a");
Which gives us the following
97
So to get the range from a to z we can try this

$random_letter_lcase = chr(rand(ord("a"), ord("z")));

Here is our complete script

<?php
srand(time());
$random_letter_lcase = chr(rand(ord("a"), ord("z")));
$random_letter_ucase = chr(rand(ord("A"), ord("Z")));
echo ("your letter is $random_letter_lcase");
echo ("<br>");
echo ("your letter is $random_letter_ucase");
?>