While solving mathematical problems we often come across questions which requires the value of π. Manually inserting the value of PI (π) can be time-consuming and erroneous. It is also not considered as a good programming practice. To solve this issue a built-in function of PHP pi() comes to aid.

The pi() function in PHP is used to return the value of π . Also, M_PI is a named constant PHP which gives identical value to that of returned by pi() function. It is slightly faster than the pi() function.

Some other predefined named constants related to π are :

**M_PI_2 :**It describes π/2. Its value is 1.570796.**M_PI_4 :**It describes π/4. Its value is 0.785398.**M_1_PI :**It describes 1/π. Its value is 0.318309.**M_PI :**It describes π. Its value is 3.141592.**M_2_PI :**It describes 2/π. Its value is 0.636619.**M_SQRTPI :**It describes sqrt(π). Its value is 1.772453.**M_2_SQRTPI :**It describes 2/sqrt(π). Its value is 1.128379.

**Syntax:**

floatpi()

**Parameters**: This function does not accept any parameter.

**Return Value:** It returns a floating point value which is an approximate value of PI.

Examples:

Input : echo(pi()) Output : 3.1415926535898 Input : echo M_PI Output : 3.1415926535898

Below programs illustrate the pi() function in PHP:

- When pi() function is used:
`<?php`

`echo`

`(pi());`

`?>`

Output:

3.1415926535898

- When M_PI is used for finding the value of PI:
`<?php`

`echo`

`M_PI;`

`?>`

Output:

3.1415926535898

**Important Points To Note:**

- The pi() function can be used to return the value of π.
- M_PI is a named constant which is slightly faster than the pi() function.
- pi() function returns the value of pi as float.

**Reference**:

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

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