php man page for isset

ISSET(3)								 1								  ISSET(3)

isset - Determine if a variable is set and is not NULL

SYNOPSIS
       bool isset (mixed  $var, [mixed	$...])

DESCRIPTION
	Determine if a variable is set and is not NULL.

	If  a  variable has been unset with unset(3), it will no longer be set. isset(3) will return FALSE if testing a variable that has been set
       to NULL. Also note that a null character ( "") is not equivalent to the PHP NULL constant.

	If multiple parameters are supplied then isset(3) will return TRUE only if all of the parameters are set. Evaluation  goes  from  left	to
       right and stops as soon as an unset variable is encountered.

PARAMETERS
	      o $var
		- The variable to be checked.

	      o $...
		- Another variable ...

RETURN VALUES
	Returns TRUE if $var exists and has value other than NULL, FALSE otherwise.

CHANGELOG
       +--------+---------------------------------------------------+
       |Version |						    |
       |	|						    |
       |	|		     Description		    |
       |	|						    |
       +--------+---------------------------------------------------+
       | 5.4.0	|						    |
       |	|						    |
       |	|  Checking  non-numeric  offsets  of  strings	now |
       |	| returns FALSE.				    |
       |	|						    |
       +--------+---------------------------------------------------+
EXAMPLES
       Example #1

	      isset(3) Examples

	      <?php

	      $var = '';

	      // This will evaluate to TRUE so the text will be printed.
	      if (isset($var)) {
		  echo "This var is set so I will print.";
	      }

	      // In the next examples we'll use var_dump to output
	      // the return value of isset().

	      $a = "test";
	      $b = "anothertest";

	      var_dump(isset($a));	// TRUE
	      var_dump(isset($a, $b)); // TRUE

	      unset ($a);

	      var_dump(isset($a));     // FALSE
	      var_dump(isset($a, $b)); // FALSE

	      $foo = NULL;
	      var_dump(isset($foo));   // FALSE

	      ?>

	This also work for elements in arrays:

       <?php

       $a = array ('test' => 1, 'hello' => NULL, 'pie' => array('a' => 'apple'));

       var_dump(isset($a['test']));	       // TRUE
       var_dump(isset($a['foo']));	       // FALSE
       var_dump(isset($a['hello']));	       // FALSE

       // The key 'hello' equals NULL so is considered unset
       // If you want to check for NULL key values then try:
       var_dump(array_key_exists('hello', $a)); // TRUE

       // Checking deeper array values
       var_dump(isset($a['pie']['a']));        // TRUE
       var_dump(isset($a['pie']['b']));        // FALSE
       var_dump(isset($a['cake']['a']['b']));  // FALSE

       ?>

       Example #2

	      isset(3) on String Offsets

	       PHP 5.4 changes how isset(3) behaves when passed string offsets.

	      <?php
	      $expected_array_got_string = 'somestring';
	      var_dump(isset($expected_array_got_string['some_key']));
	      var_dump(isset($expected_array_got_string[0]));
	      var_dump(isset($expected_array_got_string['0']));
	      var_dump(isset($expected_array_got_string[0.5]));
	      var_dump(isset($expected_array_got_string['0.5']));
	      var_dump(isset($expected_array_got_string['0 Mostel']));
	      ?>

	      Output of the above example in PHP 5.3:

	      bool(true)
	      bool(true)
	      bool(true)
	      bool(true)
	      bool(true)
	      bool(true)

	      Output of the above example in PHP 5.4:

	      bool(false)
	      bool(true)
	      bool(true)
	      bool(true)
	      bool(false)
	      bool(false)

NOTES
       Warning

	      isset(3) only works with variables as passing anything else will result in a parse error. For checking if constants are set use  the
	      defined(3) function.

       Note

	      Because this is a language construct and not a function, it cannot be called using variable functions.

       Note

	       When using isset(3) on inaccessible object properties, the __isset() overloading method will be called, if declared.

SEE ALSO
       empty(3), __isset(), unset(3), defined(3), the type comparison tables, array_key_exists(3), is_null(3), the error control @ operator.

PHP Documentation Group 														  ISSET(3)