Here is the solution to Practice 9:

<?php

function key_of_val($search, $some_ar) {
  $result = "Sorry couldn't find \"$search\".  ";
  foreach ($some_ar as $key => $val) {
      if ($val == $search) {
	$result = "The key of \"$search\" is \"$key\".  ";
      }
  }
  return $result;
}

function flip_it($some_ar) {
  $result = array();
  foreach ($some_ar as $key => $val) {
      $result["$val"] = $key;
  }
  return $result;
}


$some_ar = array(
		 "greeting" => "hello",
		 "farewell_message" => "goodbye"
		 );

print key_of_val("hello", $some_ar);
print "<br>";
print key_of_val("greeting", $some_ar);
print "<br>";

$some_ar = flip_it($some_ar);
print "<p>Inverted the array";
print "<p>";

print key_of_val("hello", $some_ar);
print "<br>";
print key_of_val("greeting", $some_ar);
print "<br>";
?>

Note that as of PHP4 there is a function called array_flip which does what flip_it() does.