Finding the duplicate number with O(n) time complexity

Finding the duplicate number

  function findDuplicateNumber( $number )
  {
      $count = count($number);
      $duplicateNumber = 0;
      
      for( $i = 0; $i < $count; $i++ )
      {
          if( $number[abs($number[$i])] >= 0 ) {
              $number[abs($number[$i])] = -$number[abs($number[$i])];
          } else {
                $duplicateNumber = abs($number[$i]);
                break;
          }
      }
      
      return $duplicateNumber;
  }
  
  $number = array(3,4,5,1,2,3);
  
  echo findDuplicateNumber($number);