| Value of variable ($var) | if($var) | isset | empty | is_null |
|---|---|---|---|---|
| $var=1; | true | true | false | false |
| $var="php"; | true | true | false | false |
| $var=" "; (space) | true | true | false | false |
| $var=TRUE; | true | true | false | false |
| $var=0; | false | true | true | false |
| $var=0.0; | false | true | true | false |
| $var="0"; | false | true | true | false |
| $var=""; (an empty string) | false | true | true | false |
| $var=FALSE; | false | true | true | false |
| $var=NULL; | false | false | true | true |
| $var (a variable declared, but without a value) | *false* | false | true | *true* |
| undefined | *false* | false | true | *true* |
| $var=array(1); | true | true | false | false |
| $var=array(); (an empty array) | false | true | true | false |
* "Notice: Undefined variable: ..." also occurs.