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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 asin() function. The asin() function in PHP is used to find the arc sine of a number in radians.
We already have discussed about PHP | sin() Function. The asin() function is the complementary function of sin().
The parameter passed in the asin() function should specify a number in the range -1 to 1.

Syntax:

`float asin(\$value)`

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

Examples:

```Input : asin(0)
Output : 0

Input : asin(1)
Output : 1.5707963267949

Input : asin(-1)
Output : -1.5707963267949

Input : asin(2)
Output : NaN
```

Below programs illustrate asin() function in PHP:

• Passing 0 as a parameter:

 `       `

Output:

`0`
• Passing 1 as a parameter:

 `       `

Output:

`1.5707963267949`
• Passing -1 as a parameter:

 `       `

Output:

`-1.5707963267949`
• Passing 2 as a parameter:

 `       `

Output:

`NaN`

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