<?php
$str = "Jackdaws love my bg sphinx of quart.";
preg_match_all("/[A-z]{1}/", strtolower($str), $letters);
$counters = [];
foreach($letters[0] as $key=>$value){
$counters[$value] = 1;
}
if(count($counters) == 25){
echo "Является.";
}else{
echo "Не является.";
}