Issue while generating unique alphabets in PHP

Am trying to generate a unique value in PHP where the first and last letters should be J and 1 respectively while the middle characters should be a series of aplhabetical letters and the whole value should be 7 in total eg JWERYH1 Please assist?

$let = chr(rand(65, 90));

$all = 'J' . $let . '1';

dd($all);