isset(3) [php man page]
ISSET(3) 1 ISSET(3) isset - Determine if a variable is set and is not NULL SYNOPSIS
bool isset (mixed $var, [mixed $...]) DESCRIPTION
Determine if a variable is set and is not NULL. If a variable has been unset with unset(3), it will no longer be set. isset(3) will return FALSE if testing a variable that has been set to NULL. Also note that a null character ( "