<?php
$str = "Jackdaws love my bg sphinx of quarfddft.";
preg_match_all("/[A-z]{1}/", strtolower($str), $letters);

$counters = [];
foreach($letters[0] as $key=>$value){
	$counters[$value] = 1;
}

if(count($counters) == 25){
	echo "Является.";
}else{
	echo "Не является.";
}