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 acos() function. The *acos()* function in PHP is used to find the arc cosine of a number in radians.

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

The parameter passed in the acos() function should specify a number in the range -1 to 1.

**Syntax:**

floatacos($value)

**Parameters:** This function accepts a single parameter $value. It is the number whose arc cosine 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 cosine value of number passed to it as argument.

Examples:

Input : 0 Output : 1.5707963267949 Input : 1 Output : 0 Input : -1 Output : 3.1415926535898 Input : 2 Output : NaN

- Passing 0 as a parameter:
`<?php`

`echo`

`(`

`acos`

`(0));`

`?>`

Output:

1.5707963267949

- Passing 1 as a parameter:
`<?php`

`echo`

`(`

`acos`

`(1));`

`?>`

Output:

0

- Passing -1 as a parameter:
`<?php`

`echo`

`(`

`acos`

`(-1));`

`?>`

Output:

3.1415926535898

- Passing 2 as a parameter. As 2 is outside the range [-1,1], the function will return NaN :
`<?php`

`echo`

`(`

`acos`

`(2));`

`?>`

Output:

NaN

**Reference**:

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

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