StringsInPHP - //Problem 2. Count of Letters

//Problem 2.    Count of Letters
//Write a program that reads a list of letters and prints for each letter how many times it appears in the list. The letters should be //listed in alphabetical order. Use the input and output format from the examples below.

<!DOCTYPE html>
<html>
    <head>
        <meta charset="UTF-8">
        <title></title>
    </head>
    <body>
        <form method="post" action="CountLetters.php">
            <input type="text" name="str1">
            <input type="submit" value="GO">
        </form>
    </body>
</html>
<?php
$string1=$_POST['str1'];
$arr1 = explode(" ", $string1);
sort($arr1);
for($i=0;$i<=count($arr1)-1;$i++){
    if ($arr1[$i]==$arr1[$i+1]){
        if (!(isset($output[$arr1[$i]]))) {
        $output[$arr1[$i]]=1;
        }
        $output[$arr1[$i]]+=1;
    }
}
foreach($output as $k=>$v){
    echo $k.'->'.$v.'<br>';
}
?>