   #PHP Manual Variable Functions is_string print_r

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                                     isset

   (PHP 3, PHP 4 )
   isset -- Determine whether a variable is set

Description

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

   Returns TRUE if var exists; FALSE otherwise.

   If a variable has been unset with unset(), it will no longer be set.
   isset() will return FALSE if testing a variable that has been set to
   NULL. Also note that a NULL byte ("\0") is not equivalent to the PHP
   NULL constant.

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

   <?php
   $var = '';
   // This will evaluate to &true; so the text will be printed.
   if (isset($var)) {
       echo "This var is set 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
   $a = array ('test' => 1, 'hello' => NULL);
   var_dump(isset($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
   ?>

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

   See also empty(), unset(), defined(), the type comparison tables,
   array_key_exists(), and the error control @ operator.
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