Code Wars counting duplicates challenge done in PHP

countingDuplicates.php

<?php
function duplicateCount($text) {
  $ret  = 0;
  $a    = [];
  $t    = str_split( strtolower( $text ) );
  $l    = count($t);
  for($i=0; $i<$l; $i++){
    $c = $t[$i];
    if ( array_key_exists($c, $t) ){
      if( $t[$c] === 1 )
        $ret += 1;
      $t[$c] += 1;
    }
    else{
      $t[$c] = 1;
    }
  }//end i-for
  return $ret;
}

// PHPUnit Test Examples:
assert(0 == duplicateCount("") );
assert(0 == duplicateCount("abcde") );
assert(2 == duplicateCount("aabbcde") );
assert(2 == duplicateCount("aabBcde") );  //should ignore case
assert(1 == duplicateCount("Indivisibility") );
//Characters may not be adjacent
assert(2 == duplicateCount("Indivisibilities"));

?>

Thank you!