PHP：计算滞后

I'm trying to calculate if heater need start or stop.

But I have some difficult with the control code:

$hysteresis = 0.5;

        $min_limit = $target_temperature-$hysteresis;
        $max_limit = $target_temperature+$hysteresis;

        if ($current_temperature<=$min_limit) {
            echo 'eseguito parte 1';
            return true;
        }

        if ($current_temperature>=$max_limit) {
            echo 'eseguito parte 2';
            return false;
        }

E.g for a target temperature == 21, heater need to start at 20.5 and stop at 21.5.

But with current_temperature == 21.2 (for example) neither of the code is executed.

How can I intercept also the intermediate case?

Thank you

EDIT:

Thank to the comments of Mike and Federkun:

if ( ( ($current_temperature<=$min_limit)===false ) && ( ($current_temperature>=$max_limit)===false ) ) {
            return true;        
        }