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<?php  function factorial_of_a_number($n)  {    if($n ==0)      {         return 1;      }    else       {            return $n * factorial_of_a_number($n-1);      }      }  print_r(factorial_of_a_number(4));  ?>   - See more at: http://www.w3resource.com/php-exercises/php-function-exercises.php#sthash.MaMd00mR.dpuf
based on kYofQ
Output for 5.4.0 - 5.4.32
<?php  function factorial_of_a_number($n)  {    if($n ==0)      {         return 1;      }    else       {            return $n * factorial_of_a_number($n-1);      }      }  print_r(factorial_of_a_number(4));  ?>   - See more at: http://www.w3resource.com/php-exercises/php-function-exercises.php#sthash.MaMd00mR.dpuf