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isset
(PHP 4, PHP 5, PHP 7)
isset — Determine if a variable is declared and is different than NULL
Description
Determine if a variable is considered set,
this means if a variable is declared and is different than NULL.
If a variable has been unset with the unset() function, it is no longer considered to be set.
isset() will return FALSE when checking a
variable that has been assigned to NULL.
Also note that a null character ("\0") is not
equivalent to the PHP NULL constant.
If multiple parameters are supplied then isset()
will return TRUE only if all of the parameters are considered set.
Evaluation goes from left to right and stops as soon as an unset
variable is encountered.
Examples
Example #1 isset() 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() on String Offsets
PHP 5.4 changes how isset() 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
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.
Note: Because this is a language construct and not a function, it cannot be called using variable functions.
Note:
When using isset() on inaccessible object properties, the __isset() overloading method will be called, if declared.
See Also
- empty() - Determine whether a variable is empty
- __isset()
- unset() - Unset a given variable
- defined() - Checks whether a given named constant exists
- the type comparison tables
- array_key_exists() - Checks if the given key or index exists in the array
- is_null() - Finds whether a variable is NULL
- the error control @ operator
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