# PHP: For loop

A `while` loop works perfectly for situations where the number of iterations isn't known in advance, e.g., finding a prime number. When we do know the number of iterations, it's better to use a `for` loop.

Let's have a look at a function that reverses strings using a `for` loop:

``````<?php

function reverseString(\$str)
{
\$result = '';

for (\$i = 0; \$i < strlen(\$str); \$i += 1) {
\$currentChar = \$str[\$i];
\$result = "{\$currentChar}{\$result}";
}

return \$result;
}
``````

https://replit.com/@hexlet/php-basics-for-reversestring

You can read it like this: «the loop with index `\$i` repeats until `\$i < strlen(\$str)` and after each step increases `\$i` by 1».

1. The initial counter value. This code runs exactly once before the first iteration.
2. Predicate condition to decide when cycle repeats. It's executed on each iteration. This is exactly what we have with `while`
3. Description of the counter change. This code is executed at the end of each iteration.

Other than that, it works exactly the same as a `while` loop.

## Instructions

Implement a function called `sumOfSeries()`, which counts the sum of a series of integers. A series is given using two numbers, a start number and an end number.

The function takes two number arguments and returns the sum of the series. For example, if you pass `4` and `7` as arguments, the sum will be `4 + 5 + 6 + 7 = 22`.

``````<?php

sumOfSeries(1, 2); // 3
sumOfSeries(1, 3); // 6
sumOfSeries(4, 7); // 22
``````

If the initial and final numbers are equal, the result will be the same number:

``````<?php

sumOfSeries(1, 1); // 1
``````
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 🙄

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