wordpress - PHP return output based on zero, empty or value -

i have simple requirement proving difficult. i'm using wordpress store meta value 'session_capacity', can have following possible values

• 0
• integer e.g. 99
• nothing, left blank

the problem when value stored zero, can't figure out setup use echo different statement based on whether it's 0 or empty.

this desired output...

if (zero) // elseif (integer e.g. 99) // elseif (blank) // something

actual function...

function get_session_capacity($post_id){     // total capacity of session     $session_capacity = get_post_meta($post_id, 'session_capacity', true);     // meta value stored string converting integer     $session_capacity = intval($session_capacity);      if($session_capacity === 0)          return 'you cannot book session';     elseif(empty($session_capacity))         return default_session_capacity();     else         return $session_capacity; } 

you can check if $session_capacity set or not, , set accordingly

$session_capacity = isset($session_capacity)? intval($session_capacity) : null; 

or better

$session_capacity = (isset($session_capacity) && !empty($session_capacity))? intval($session_capacity) : null; 

the latter works when variable set empty string. doesn't give 0 when $session_capacity not set (blank)