Here’s the function:

<?php
function guessGender($relation)
{
 $gen = "";
 $relation = ucwords(strtolower($relation));
 $relations = array(
      "Male" => array("Husband", "Son", "Father", "Brother"),
      "Female" => array("Wife", "Daughter", "Mother", "Sister")
 );

 foreach($relations as $gender => $relationType){
      if(in_array($relation, $relationType)){
           $gen = $gender;
      }
 }

 return $gen;
}
?>

For the ease of your understanding, I have linked each PHP function, language construct and other statements to Official PHP manual. Simply click on the provided links (in above code) to see the detailed information on each.