Complementing (as an algebraic curiosity) the @deceze answer with the @ operator, and indicating cases where is "better" to use @ ... Not really better if you need (no log and) micro-performance optimization:
array_key_exists: is true if a key exists in an array; isset: is true if the key/variable exists and is not null [faster than array_key_exists]; @$array['key']: is true if the key/variable exists and is not (null or '' or 0); [so much slower?]
$a = array('k1' => 'HELLO', 'k2' => null, 'k3' => '', 'k4' => 0);
print isset($a['k1'])? "OK $a[k1].": 'NO VALUE.'; // OK
print array_key_exists('k1', $a)? "OK $a[k1].": 'NO VALUE.'; // OK
print @$a['k1']? "OK $a[k1].": 'NO VALUE.'; // OK
// outputs OK HELLO. OK HELLO. OK HELLO.
print isset($a['k2'])? "OK $a[k2].": 'NO VALUE.'; // NO
print array_key_exists('k2', $a)? "OK $a[k2].": 'NO VALUE.'; // OK
print @$a['k2']? "OK $a[k2].": 'NO VALUE.'; // NO
// outputs NO VALUE. OK . NO VALUE.
print isset($a['k3'])? "OK $a[k3].": 'NO VALUE.'; // OK
print array_key_exists('k3', $a)? "OK $a[k3].": 'NO VALUE.'; // OK
print @$a['k3']? "OK $a[k3].": 'NO VALUE.'; // NO
// outputs OK . OK . NO VALUE.
print isset($a['k4'])? "OK $a[k4].": 'NO VALUE.'; // OK
print array_key_exists('k4', $a)? "OK $a[k4].": 'NO VALUE.'; // OK
print @$a['k4']? "OK $a[k4].": 'NO VALUE.'; // NO
// outputs OK 0. OK 0. NO VALUE
PS: you can change/correct/complement this text, it is a Wiki.
