How do I count comma-separated values in PHP?

You need to explode $schools into an actual array:

$schools = $_SESSION['sarray'];
$schools_array = explode(",", $schools);
$result = count($schools_array);

if you just need the count, and are 100% sure it's a clean comma separated list, you could also use substr_count() which may be marginally faster and, more importantly, easier on memory with very large sets of data:

$result = substr_count( $_SESSION['sarray'], ",") +1; 
 // add 1 if list is always a,b,c;

Should be

$result = count(explode(',',$schools));

Actually, its simpler than that:

$count = substr_count($schools, ',') + 1;