php man page for usort

USORT(3)								 1								  USORT(3)

usort - Sort an array by values using a user-defined comparison function

SYNOPSIS
       bool usort (array  &$array, callable  $value_compare_func)

DESCRIPTION
	This function will sort an array by its values using a user-supplied comparison function. If the array you wish to sort needs to be sorted
       by some non-trivial criteria, you should use this function.

       Note

	       If two members compare as equal, their relative order in the sorted array is undefined.

       Note

	      This function assigns new keys to the elements in $array. It will remove any existing keys that may have been assigned, rather  than
	      just reordering the keys.

PARAMETERS
	      o $array
		- The input array.

	      o $value_compare_func
		-  The comparison function must return an integer less than, equal to, or greater than zero if the first argument is considered to
		be respectively less than, equal to, or greater than the second.
		int callback (mixed  $a, mixed	$b)

	      Caution

		      Returning non-integer values from the comparison function, such as float, will result in an internal cast to integer of  the
		     callback's  return  value. So values such as 0.99 and 0.1 will both be cast to an integer value of 0, which will compare such
		     values as equal.

RETURN VALUES
	Returns TRUE on success or FALSE on failure.

EXAMPLES
       Example #1

	      usort(3) example

	      <?php
	      function cmp($a, $b)
	      {
		  if ($a == $b) {
		      return 0;
		  }
		  return ($a < $b) ? -1 : 1;
	      }

	      $a = array(3, 2, 5, 6, 1);

	      usort($a, "cmp");

	      foreach ($a as $key => $value) {
		  echo "$key: $value
";
	      }
	      ?>

	      The above example will output:

	      0: 1
	      1: 2
	      2: 3
	      3: 5
	      4: 6

       Note

	       Obviously in this trivial case the sort(3) function would be more appropriate.

       Example #2

	      usort(3) example using multi-dimensional array

	      <?php
	      function cmp($a, $b)
	      {
		  return strcmp($a["fruit"], $b["fruit"]);
	      }

	      $fruits[0]["fruit"] = "lemons";
	      $fruits[1]["fruit"] = "apples";
	      $fruits[2]["fruit"] = "grapes";

	      usort($fruits, "cmp");

	      while (list($key, $value) = each($fruits)) {
		  echo "$fruits[$key]: " . $value["fruit"] . "
";
	      }
	      ?>

	       When sorting a multi-dimensional array, $a and $b contain references to the first index of the array.

	      The above example will output:

	      $fruits[0]: apples
	      $fruits[1]: grapes
	      $fruits[2]: lemons

       Example #3

	      usort(3) example using a member function of an object

	      <?php
	      class TestObj {
		  var $name;

		  function TestObj($name)
		  {
		      $this->name = $name;
		  }

		  /* This is the static comparing function: */
		  static function cmp_obj($a, $b)
		  {
		      $al = strtolower($a->name);
		      $bl = strtolower($b->name);
		      if ($al == $bl) {
			  return 0;
		      }
		      return ($al > $bl) ? +1 : -1;
		  }
	      }

	      $a[] = new TestObj("c");
	      $a[] = new TestObj("b");
	      $a[] = new TestObj("d");

	      usort($a, array("TestObj", "cmp_obj"));

	      foreach ($a as $item) {
		  echo $item->name . "
";
	      }
	      ?>

	      The above example will output:

	      b
	      c
	      d

       Example #4

	      usort(3) example using a closure to sort a multi-dimensional array

	      <?php
	      $array[0] = array('key_a' => 'z', 'key_b' => 'c');
	      $array[1] = array('key_a' => 'x', 'key_b' => 'b');
	      $array[2] = array('key_a' => 'y', 'key_b' => 'a');

	      function build_sorter($key) {
		  return function ($a, $b) use ($key) {
		      return strnatcmp($a[$key], $b[$key]);
		  };
	      }

	      usort($array, build_sorter('key_b'));

	      foreach ($array as $item) {
		  echo $item['key_a'] . ', ' . $item['key_b'] . "
";
	      }
	      ?>

	      The above example will output:

	      y, a
	      x, b
	      z, c

SEE ALSO
       uasort(3), The comparison of array sorting functions.

PHP Documentation Group 														  USORT(3)