# PHP: Arithmetic Operations

At the most basic level, computers only use numbers. Even in high-level language applications, there are many numbers and operations with them. Fortunately, all you need to know to get started is basic arithmetic, so let's start there.

To add two numbers in math we write something like `3 + 4`. The same goes for programming. Here's a program that adds two numbers:

``````<?php

// Don't forget the semicolon at the end, because every line in the code is an instruction.
3 + 4;
``````

Remember comments? In the code above, after `//` you can see a comment from the teacher.

Arithmetic in programming is virtually the same as school arithmetic.

The instruction `3 + 4;` will force the interpreter to add the numbers and find the result. This program will work, but it's pointless, because «we're not really giving the interpreter a command, we're just telling it to calculate three plus four». In real life, you need to do more than just tell the interpreter about the mathematical expression on its own. For example, when creating an online store, it's not enough to ask the interpreter to calculate the cost of the goods in their shopping cart, you need to ask it to calculate the cost AND show the price to the customer.

We need to ask the interpreter to add `3 + 4` AND give it a command to do something with the result. For example, print it:

``````<?php

// again, don't forget the semicolon at the end of the line
print_r(3 + 4);
``````

After we run the program, the result will appear on the screen:

```7
```

Besides addition, we can also use the following operations:

• `*` — multiplication
• `/` — division
• `-` — subtraction
• `%`remainder
• `**` — exponentiation

Now let’s print the result of a division, and then the result of an exponentiation:

``````<?php

print_r(8 / 2);  // => 4
print_r(3 ** 2); // => 9
``````

Sometimes, for convenience, we'll show the result of running lines of code in the comments like this: `=> RESULT`. For example, `// => 4`.

The first statement will print `4` (since 8 / 2 is 4), and the second statement will print 9 (since 32 is 9).

## Instructions

Divide `81` by `9` and display the result.

The exercise doesn't pass checking. What to do? 😶

• Be sure to attach the test output, without it it's almost impossible to figure out what went wrong, even if you show your code. It's complicated for developers to execute code in their heads, but having a mistake before their eyes most probably will be helpful.
In my environment the code works, but not here 🤨

Tests are designed so that they test the solution in different ways and against different data. Often the solution works with one kind of input data but doesn't work with others. Check the «Tests» tab to figure this out, you can find hints at the error output.

My code is different from the teacher's one 🤔

It's fine. 🙆 One task in programming can be solved in many different ways. If your code passed all tests, it complies with the task conditions.

In some rare cases, the solution may be adjusted to the tests, but this can be seen immediately.

I've read the lessons but nothing is clear 🙄

It's hard to make educational materials that will suit everyone. We do our best but there is always something to improve. If you see a material that is not clear to you, describe the problem in “Discussions”. It will be great if you'll write unclear points in the question form. Usually, we need a few days for corrections.

By the way, you can participate in courses improvement. There is a link below to the lessons course code which you can edit right in your browser.

## Tips

• Always indent arithmetic operators with spaces between the numbers (operands). It's good form when programming. Therefore, in our examples, we have `print_r(3 + 4)` rather than `print_r(3+4)`.

• Dividing by zero causes, unsurprisingly, a `Division by zero` error. To avoid it, you need to know about conditional constructs (you'll learn about them in future

• Type your code between the`// BEGIN` and `// END` lines in the editor.

If you got stuck and don't know what to do, you can ask a question in our community