# PHP Math - acos() Function

The PHP Math acos() function returns arc cosine of a value. The return value will be in the range 0 through 𝜋. In special cases it returns the following:

• If the argument is NAN or its absolute value is greater than 1, then the result is NAN.

Note: acos() is the inverse of cos().

### Syntax

```acos(number)
```

### Parameters

 `number` `Required. `Specify the value.

### Return Value

Returns the arc cosine of the value.

### Example:

In the example below, acos() function is used to find out the arc cosine of a given value.

```<?php
echo "acos(-1) = ".acos(-1)."\n";
echo "acos(-0.5) = ".acos(-0.5)."\n";
echo "acos(0) = ".acos(0)."\n";
echo "acos(0.5) = ".acos(0.5)."\n";
echo "acos(1) = ".acos(1)."\n";
echo "acos(2) = ".acos(2)."\n";
echo "acos(NAN) = ".acos(NAN)."\n";
?>
```

The output of the above code will be:

```acos(-1) = 3.1415926535898
acos(-0.5) = 2.0943951023932
acos(0) = 1.5707963267949
acos(0.5) = 1.0471975511966
acos(1) = 0
acos(2) = NAN
acos(NAN) = NAN
```

❮ PHP Math Functions

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