<?php $a = null; if(isset($a)){ echo "is set"."\n"; } else{ echo "not set"."\n"; } if(empty($a)){ echo "is empty"."\n"; } else{ echo "not empty"."\n"; } ?> not set is empty
<?php $a = 0; if(isset($a)){ echo "is set"."\n"; } else{ echo "not set"."\n"; } if(empty($a)){ echo "is empty"."\n"; } else{ echo "not empty"."\n"; } ?> is set is empty