PHP Program to Find Area of Circle with Example

To find the area of a circle in PHP, use the formula area = pi() * radius * radius where pi() gives the value of π and radius is the circle's radius.

Examples

Inputradius = 0

OutputArea of circle with radius 0 is 0

Inputradius = 5

OutputArea of circle with radius 5 is 78.539816339745

Inputradius = 10.5

OutputArea of circle with radius 10.5 is 346.360590058274

How to Think About It

To find the area of a circle, you need the radius. The formula is area = π times radius squared. You get π using PHP's built-in function pi(). Multiply pi by radius times radius to get the area.

Algorithm

1

Get the radius value from the user or set it in the program

2

Calculate the area using the formula area = pi() * radius * radius

3

Print or return the calculated area

Code

php

<?php
$radius = 5;
$area = pi() * $radius * $radius;
echo "Area of circle with radius $radius is $area";
?>

Output

Area of circle with radius 5 is 78.539816339745

Dry Run

Let's trace the program with radius = 5 through the code

1

Set radius

radius = 5

2

Calculate area

area = pi() * 5 * 5 = 3.1415926535898 * 25 = 78.539816339745

3

Print result

Output: Area of circle with radius 5 is 78.539816339745

Step	Variable	Value
1	radius	5
2	area	78.539816339745
3	output	Area of circle with radius 5 is 78.539816339745

Why This Works

Step 1: Using pi() function

The pi() function returns the value of π, which is needed to calculate the circle's area.

Step 2: Calculating area

Multiply π by the radius squared (radius * radius) to get the area.

Step 3: Displaying result

Print the area with a message so the user knows the radius and the calculated area.

Alternative Approaches

Using a function to calculate area

php

<?php
function circleArea($r) {
    return pi() * $r * $r;
}
$radius = 7;
echo "Area of circle with radius $radius is " . circleArea($radius);
?>

This approach makes the calculation reusable by putting it inside a function.

Using pow() function for exponentiation

php

<?php
$radius = 3;
$area = pi() * pow($radius, 2);
echo "Area of circle with radius $radius is $area";
?>

Using <code>pow()</code> makes the code clearer when squaring the radius.

Complexity: O(1) time, O(1) space

Time Complexity

The calculation uses a fixed number of operations regardless of input size, so it is constant time O(1).

Space Complexity

Only a few variables are used to store radius and area, so space is constant O(1).

Which Approach is Fastest?

All approaches run in constant time; using a function adds readability but no significant speed difference.

Approach	Time	Space	Best For
Direct calculation	O(1)	O(1)	Simple quick calculation
Function method	O(1)	O(1)	Reusable code and clarity
Using pow() function	O(1)	O(1)	Clear exponentiation syntax