PHP: Does empty() validate all of these cases: null, empty string, variable not set?

[leox2201]

Does empty() function validate all of this cases:1.var=null 2.var=empty string ""/" " 3.var=not setExample for validate use (Should I add some more code for validation or empty() enough?):\[code\]if(!empty($userName)){ //some code}else{ echo "not valid user name"}\[/code\]Edit:should isset() used before empty() or empty() include isset() case?Thanks