I have to agree with bumperbox here - why create separate variables instead of using an array to store them all, as follows:

    $day = array('Mon', 'Tue', 'Wed', 'Thu', 'Fri');
    $B = array('P1B', 'P2B', 'P3B', 'P4B', 'P5B', 'P6B');
    $arr = array();

    for ($l = 0; $l < count($day); ++$l) {
        for ($k = 0; $k < count($B); ++$k) {
            $days = $day[$l];
            $week = $B[$k];
            $arr[$days . $week] = $_POST["'" . $days.$week . "'"];
        }; 
    };

Which would generate:

    Array
    (
        [MonP1B] => 
        [MonP2B] => 
        [MonP3B] => 
        ...
        [FriP4B] => 
        [FriP5B] => 
        [FriP6B] => 
    )

And which you can access via `$arr['FriP5B']` for example. 

Now, as to how you can improve your code, see this:

    $days_of_the_week = array('Mon', 'Tue', 'Wed', 'Thu', 'Fri');
    $number_of_days = count($days_of_the_week);
    $rooms = array('P1B', 'P2B', 'P3B', 'P4B', 'P5B', 'P6B');
    $number_of_rooms = count($rooms);
    $booked_day_room = array();

    for ($day = 0; $day < $number_of_days; ++$day) {
        for ($room = 0; $room < $number_of_rooms; ++$room) {
            $days_room = $days_of_the_week[$day] . $rooms[$room];
            $booked_day_room[$days_room] = isset($_POST[$days_room]) ?: $_POST[$days_room];
        };
    };

    print_r($booked_day_room);

What we did:

1. [Used descriptive variable names](https://softwareengineering.stackexchange.com/a/176585)
2. [Cached the `count()` function calls](http://stackoverflow.com/a/9714237/1753963)
3. Stored everything in 1 array, so we don't have to chase 100 different variables

Why are we using an array over multiple variables? Multiple variables might be a bit faster, and they might use more or less the same memory, however from a code readability standpoint, you want one array as it is easier to keep track of in the code.