is isset($_SESSION['admin']) a dangerous way to grant access?
If (for argument sake) 'admin-access' was granted in php with:

    if (isset($_SESSION['admin']))  // this session would be set
    { // grant access; }            // after a successful login
    else { //redirect ;}

Would this be a particularly easy thing to bypass and fake, if you knew what the name of the session is (in this case it is admin)? 

In other words, can someone easily fake a $_SESSION, if all a script calls for is the session to be 'set'?
is isset($_SESSION['admin']) a dangerous way to grant access?
===

If (for argument sake) 'admin-access' was granted in php with:

    if (isset($_SESSION['admin']))  // this session would be set
    { // grant access; }            // after a successful login
    else { //redirect ;}

Would this be a particularly easy thing to bypass and fake, if you knew what the name of the session is (in this case it is admin)? 

In other words, can someone easily fake a $_SESSION, if all a script calls for is the session to be 'set'?