检查PHP时出错

I'm working on expanding my PHP skills and I'm trying to write a simple program that calculates volume and checks to make sure all the numerical values are from 0-20, but for some reason I can't get my program to recognize incorrect numbers.

    <?php

//read deep end
$deepEnd = $_POST['deepEnd'];

//read shallow end
$shallowEnd = $_POST['shallowEnd'];

     //read length
$length = $_POST['length'];

//read width
$width = $_POST['width'];

$flag = false;
$min = 0;
$max = 20;

if($deepEnd < $min && $deepEnd > $max){
        $flag = true;
}
if($shallowEnd < $min && $shallowEnd > $max){
        $flag = true;
    }
if($length < $min && $length > $max){
        $flag = true;
}
if($width < $min && $width > $max){
         $flag = true;
 }

$volume = 0.5*($deepEnd+$shallowEnd)*$length*$width*7.5;

echo "<h2>Volume Calculator</h2>";

if($flag == 'true'){
        echo "<p>Incorrect value input. Please enter 0-20.</p>
";
} else {
        echo "<p>Your volume is: ".$volume."  gallons.</p>
";
}
?>

My input is being taken from my HTML program. The <input type='number'> so I'm confused why my if statements aren't interpreting it correctly.

You are comparing each variable if it is less than $min and more than $max. Unless your $min is greater than $max, your code wouldn't work.

Example:

$min = 0;
$max = 10;
$x = 5;
if ($x < $min && $x > $max) { // this will always evaluate as false
    // won't execute here
}

To solve your problem, which is to check if the value is out of scope of the $min or $max, you should use the || operator.

Example:

$min = 0;
$max = 10;
$x = -5;
if ($x < $min || $x > $max) { // true
    // do something
}

It seems to me your logic is wrong, in the first check:-

if($deepEnd < $min && $deepEnd > $max){
        $flag = true;
}

this will only return error if the $deepend is less than the min and greater than the max which could never really happen, testing your code :-

if($deepEnd < $min OR $deepEnd > $max){
        $flag = true;
}

worked for me