การใช้ if กำหนด ค่าระหว่างกัน ทำได้ไหมครับ หรือมีวิธี แนะนำบ้างไหม

if($students<=20){
redirect("?action=product");
}
if(($students>20)  &&  ($students <=40)) {
redirect("?action=product2");
}
if(($students>40)  &&  ($students <=60)) {
redirect("?action=product3");
}
if(($students >60)  &&  ($students<=80)) {
redirect("?action=product4");
}

if(($students>80)  &&  ($students<=100)) {
redirect("?action=product5");
}

if($students<=20){
redirect("?action=product");
} elseif(($students>20)  &&  ($students <=40)) {
redirect("?action=product2");
} elseif(($students>40)  &&  ($students <=60)) {
redirect("?action=product3");
} elseif(($students >60)  &&  ($students<=80)) {
redirect("?action=product4");
} elseif(($students>80)  &&  ($students<=100)) {
redirect("?action=product5");
}

<?php
$val = (isset($_POST['val']) ? $_POST['val'] : "");

$chk = array("1" => array("a" => "20", "b" =>"40", "r" => "product2"),
		"2" => array("a" => "40", "b" => "60", "r" => "product3"),
		"3" => array("a" => "60", "b" => "80", "r" => "product4"),
		"4" => array("a" => "80", "b" => "100", "r" => "product5"),
		"5" => array("a" => "100", "b" => "120", "r" => "product6"),
		"6" => array("a" => "120", "b" => "140", "r" => "product7")
	);

if ( $_POST ) {
	foreach ( $chk as $key => $item ) {
		if ( ($val > $item['a']) && ($val <= $item['b']) ) {
			echo $item['r'];
		}
	}
}
?>
<form method="post">
	<input type="text" name="val" /><input type="submit" value="go" />
</form>

function Ckstu($student){
switch ($student) {
 case ($student <= 20):
 echo "น้อยกว่าหรือเท่ากับ 20";
 break;
case ($student  >20 AND $student <=40):
 echo "อยู่ระหว่าง 20-40";
 break;
case ($student  >40 AND $student <=60):
 echo "อยู่ระหว่าง 40-60";
 break;
case ($student  >60 AND $student <=80):
 echo "อยู่ระหว่าง 60-80";
 break;
 }
}

echo Ckstu(15);