PHP 0 to hero pt.5 - Math and Sessions in PHP

Welcome again - Now we start with a part 4 of this crash course

I think that everyone heard, that math is useless in web dev (yes, I bolded it purposefully) - I agree and disagree at the same time.
Even though yes - stuff like linear algebra may not be the most valuable for us, yet mathematical logic or arithmetic is used very often here

And that will be our focus today - we will learn

1. Basic arithmetic operations (addition, division etc.)
2. Square roots and exponentiation
3. Logic operators
4. Sessions

It will end with a short example project - a very basic calculator
With some new, fancy stuff like sessions and switch statement

With that said - let's go

Setting up

Do you remember variables from the previous article? That's what we will use a lot today

So, start with creating new variables - let's call them a and b

$a = 25;
$b = 5;

These will be 2 values I'll use - If anything changes, it will be mentioned

But it can be whatever you want - even negative or fractions, like

$c = -4.5;

And remember - decimal fractions are written with . (dot) and not ,(comma)
With this knowledge - let's go

Basic arithmetic

This is very easy - any basic mathematical operation is done, like on paper.

To do it, we use operators. Another strange term? Not really. It's just a construct that takes an expression (one or more values) and then returns us something else, technically also becoming an expression

What does it mean - let's see example

echo $a + $b;
// 30

+ here is a operator - it connects to values and does something to them and, let's say, "merge" with them

And that's how you add 2 numbers in PHP - congrats

Rest is similar

Subtraction:

echo $a - $b;
// 20

Multiplying:

echo $a * $b;
// 125

Division:

echo $a / $b;
// 5

And last operation - modulo

echo $a % $b;

What is modulo? It's remainder of division - if you divide 3 by 2 you get 1 and a 1 as a reminder.

And that's how, you have learned everything about basic math in PHP - let's get further

Roots and exponentiation

Here, it gets a little bit trickier

To write the exponential expression - we have 2 methods

1. using ** operator

echo 5 ** 2;
// 25

1. Using pow() funtction

echo pow(5, 2);
// 25

For me - first one is better. I like simplicity that comes with operators

And (sadly) square roots don't have any operator - just sqrt() function

echo sqrt($a);
// 5

But what if we want get other root than square one - let's use old trick we have learned in high school

echo pow(8, 1/3);
// 2

Remember that if you power something to inverse of a fraction, you get a denominators root of that number

Sounds easy now - let's get to the last, and really important part

Logical Operations

To be fair - that's the most important part of this article, as we will be using logical operators very often

First - some theoretical and more mathematical info

Let's get the expression (with our previous variables)

$a + $b = 30
// 25 + 5 = 30

If we want to evaluate it's logical value - we check if 25 + 5 really equals 30

And yes, it does - so our check will return true

var_dump((bool) ($a + $b == 30));
// bool(true) 

Quick side note: This (bool) inside var_dump indicates what type of data should get returned

And it does - now, what's that strange symbol between $b and 30?

That's comparison operator named equal - this means "Do these 2 expressions have the same value?". If yes, it returns true. If not - false;

Why not =? Because = is a symbol of assignment - It would create a lot of mess, so it's standardized to ==

That's another comparison operator we have learned (after <) - Let's see them all in action

1. === - Identical. Difference between this and equal is, that it also checks type So this

var_dump((bool) ($a + $b == 30.0));
// bool(true)

Gives us true or 1 (it's the same and can be used interchangeably)

But if we use ===

var_dump((bool) ($a + $b === 30.0));
// bool(false) 

We get false (or 0 - it's the same too)

1. != - Not equal. The opposite of == - will return true if expressions don't have the same value

var_dump((bool) ($a + $b != 32));
// bool(true)

Not equal has also it's alternative - <>. But you shouldn't use it in your code. != is much more readable.

I mentioned it, as it can sometimes be seen in older codes - so you know

1. !== - Not identical. The difference here is the same as between == and ===. This operator checks also types

So this:

var_dump((bool) ($a + $b !== 30.0));
// bool(true)

Gives us true, because the types don't match

1. <, > - less than and greater than. I put them together because
   • We know one of them - it appeared in pt. 3
   • They are identical to their math equivalent

var_dump((bool) ($a + $b > 10.0));
// bool(true) 

And the same goes another way - I hope you get the point of operators?

Now last ones:

1. >= - Greater than or equal to. It will return true if expression on right side is grater or equal to one on the left side

It's math equivalent is ≥

var_dump((bool) ($a + $b >= 10)); // bool(true)
// And also
var_dump((bool) ($a + $b >= 30)); // bool(true)

1. <= - Less than or equal to. This one is the opposite of previous one

It's mathematical symbol is ≤

var_dump((bool) ($a + $b <= 10)); // bool(false)
// And also
var_dump((bool) ($a + $b <= 30)); // bool(true)

And that's it - pretty much every operator you need to know

For some further explanation - visiting PHP docs is recommended

So now, let's leave the theory and start writing our calculator

Practical Project - Calculator

So, at first start your server and create a new folder - I'll call it calculator and inside index.php

So, first thing is basic HTML template

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Calculator</title>
</head>
<body>
    <form action="calculate.php" method = "GET">
        <input type="number" name="num1" id="" />
        <input type="number" name="num2" id="" />
        <select name = "operation">
            <option value ="add">Add</option>
            <option value ="subtract">Subtract</option>
            <option value ="multiply">Multiply</option>
            <option value ="divide">Divide</option>
            <option value = "expon">Exponentiation</option>
            <option value = "root"> Square Root</option>
            <option value = "modulo">Modulo</option>
        </select>

        <button type="submit">Calculate</button>
    </form>
</body>
</html>

What is <select>? - It's this drop-down form field

It has operation as name - this works just as in any other form field.

And each <option> element represents... the option. So, content of the tag is the label in form field, but value is the value of $_GET['operation'] then in PHP

With this said - let's dive deeper

Now, action element isn't empty - so we will create a new file called calculate.php solely for PHP.

Inside a file, add PHP tag and if data is present, we will save it to additional variables and another called result - it will be useful in the end

<?php

if(!isset($_GET['num1'])){
    header("Location: index.php");
    exit();
}

$num1 = $_GET['num1'];
$num2 = $_GET['num2'];
$operation = $_GET['operation'];
$result = 0;

And now we could spend a lot of time writing if-else structures, creating really big mess (so-called spaghetti code). But there is another way

Switch statement

That's it - there is a structure, that can replace a lot of if-else code and make it clearer

Let's write and analyze it

switch($operation){
    case "add": $result = $num1 + $num2; break;
    case "subtract": $result = $num1 - $num2; break;
    case "multiply": $result = $num1*$num2; break;
    case "divide": $result = $num1/$num2; break;
    case "expon": $result = $num1**$num2; break;
    case "root": $result = sqrt($num1); break;
    case "modulo": $result = $num1%$num2; break;

    default: $result = "Invalid operation chosen"; break;
}

So, step by step

inside brackets switch takes a value which will then be compared - here $operation

And then

 case "add": $result = $num1 + $num2; break;

This can be represented as

if($operation == "add"){
   $result = $num1 + $num2;
}

And the same is done with each and every case

But what does break do? This breaks out of the statement

If $num1 = 4 and $num2 = 2 and $operation = "divide", we would have everything below case "divide" executed

So first $result would really equal 2, then 16, then again 2, then 0 and in the and we would get a message Invalid operation chosen

Sometimes it may be useful - but don't forget break; if you want to replace if-else with switch

And what does default mean? It means, that if value does not meet any condition, it will get executed

So if somehow $operations = "idk", and as it doesn't match at all, we see Invalid operation chosen

(Yes, you have to think about such edge-cases, as sometimes you can get amazed with what can people do with the website. It just has to be as idiot-proof as possible)

Now, let's display the value

In the end add

echo $result;

After submitting the form - you should see a page with a number - that's great, but wouldn't we like to have it on the same page as form?

And that's also why I've created that result variable.

So, let's create a session

First, at the beginning of each file add

session_start();

In index.php place it right above Doctype inside PHP tags

That function either starts, or resumes existing session.

So, if we start the session in index.php, then switch to calculate.php - it will get resumed

Then, we can use really important feature called Session Variables

To access them, you select a specific field in superglobal associative array called $_SESSION - You use it just like $_GET, but here you can add whatever you want without limitation

So, let's create one for $result and redirect user to calculate page

$_SESSION['result'] = $result;

header("Location: index.php");

When it's done, we can go to index.php and below form add a new PHP tag

<?php

?>

Don't forget the closing here

Now, just display it like normal variable

echo $_SESSION['result'];

Right?

Not really - If you do that, and refresh the page without sending a form, you will see this messages

Warning: Undefined variable $_SESSION in [PATH REDACTED] on line 25

Warning: Trying to access array offset on value of type null in [PATH REDACTED] on line 25

Not so cool, how do we fix it? Go to the start of calculate.php. Do you remember this?

if(!isset($_GET['num1'])){
    // Code responsible for redirection
}

That's what we have to do here too - check if the value is set, and if yes, display it

Simple, isn't it? Now, let's write it

        if(isset($_SESSION['result'])){
            echo "Result is:";
            echo $_SESSION['result'];
            exit();
        }

And it works! - no errors.
Now let's calculate something - like 5+2

It also works - well done.

That's how you have written you calculator - good job!

Conclusion

Another day, another thing learned - This was one of longer articles

Of course - code is available on GitHub for you. By the way, I realized that there is no code for part 3. It will be uploaded as well

I hope you liked it, and learned something

I am always open for your feedback - that's what comments are for.

Check out other parts of this series and see you next time

Also, merry Christmas for you all