Trigonometry is an important part of mathematics and PHP’s math functions provides us with some functions which are very useful for calculations involving trigonometry. One of such function is atan() function.

The *atan()* function in PHP is used to find the arc tangent of an argument passed to it which has as a numeric value between -Pi/2 and Pi/2 radians.

We already have discussed about PHP | tan() Function. The atan() function is the complementary function of tan().

**Syntax:**

floatatan($value)

**Parameters:** This function accepts a single parameter $value. It is the number whose arc tangent value you want to find. The value of this parameter must be in radians.

**Return Value:** It returns a floating point number which is the arc tangent value of number passed to it as argument.

Examples:

Input : atan(0.50) Output : 0.46364760900081 Input : atan(-0.50) Output : -0.46364760900081 Input : atan(5) Output : 1.373400766945 Input : atan(100) Output : 1.5607966601082

Below programs with different values of $value illustrate the atan() function in PHP:

- Passing 0.50 as a parameter:
`<?php`

`echo`

`(`

`atan`

`(0.50));`

`?>`

Output:

0.46364760900081

- Passing -0.50 as a parameter:
`<?php`

`echo`

`(`

`atan`

`(-0.50));`

`?>`

Output:

-0.46364760900081

- Passing 5 as a parameter:
`<?php`

`echo`

`(`

`atan`

`(5));`

`?>`

Output:

1.373400766945

- Passing 100 as a parameter:
`<?php`

`echo`

`(`

`atan`

`(100));`

`?>`

Output:

1.5607966601082

**Reference**:

http://php.net/manual/en/function.atan.php

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