The isset() function is used to detect whether a variable has been set and is not NULL.
If a variable has been released using unset(), then isset() will return FALSE.
If isset() is used to test a variable that is set to NULL, FALSE will be returned.
Also note that the null character (" ") is not equivalent to PHP's NULL constant.
PHP version requirements: PHP 4, PHP 5, PHP 7
grammar
bool isset ( mixed $var [, mixed $... ] )
Parameter description:
$var: The variable to be tested.
If multiple parameters are passed in at one time, isset() will only return TRUE when all parameters are set. The calculation process is from left to right and will stop immediately when an unset variable is encountered.
return value
Returns TRUE if the specified variable exists and is not NULL, otherwise returns FALSE.
Example
Example
<?php $var = ' ' ; // The result is TRUE, so the following text will be printed. if ( isset ( $var ) ) { echo " Variable is set. " . PHP_EOL ; } // In the following examples, we will use var_dump to output the return value of isset(). // 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 ?> The output is:
The variable is set. bool(true)bool(true)bool(false)bool(false)bool(false)
This also works for elements in an array:
Example
<?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 value of key 'hello' is equal to NULL, so it is considered unset // If you want to detect NULL key values, you can try the following method. var_dump ( array_key_exists ( ' hello ' , $a ) ) ; // TRUE // Deeper detection var_dump ( isset ( $a [ ' pie ' ] [ ' a ' ] ) ) ; // TRUE var_dump ( isset ( $a [ ' pie ' ] [ ' b ' ] ) ) ; // FALSE var_dump ( isset ( $a [ ' cake ' ] [ ' a ' ] [ ' b ' ] ) ) ; // FALSE ?> The output is:
bool(true)bool(false)bool(false)bool(true)bool(true)bool(false)bool(false)
Using isset() with string displacement
PHP 5.4 changes the behavior of isset() when passed a string offset.
Example
<?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 ' ] ) ) ; ?> The output of the above example in PHP 5.3:
bool(true)bool(true)bool(true)bool(true)bool(true)bool(true)
The output of the above example in PHP 5.4:
bool(false)bool(true)bool(true)bool(true)bool(false)bool(false)
