# Program to find Factorial of a number in PHP

`function factorial(\$n) { if (\$n == 0) { return 1; } else { return \$n * factorial(\$n - 1); } } // Taking input from the user \$number = readline("Enter a number: "); // Checking if the input number is a positive integer if (\$number < 0) { echo "Factorial is not defined for negative numbers."; } elseif (\$number == 0) { echo "Factorial of 0 is 1."; } else { \$result = factorial(\$number); echo "Factorial of " . \$number . " is " . \$result; }`

In this PHP program, we define a recursive function `factorial()` that calculates the factorial of a number `\$n`. The function checks if the input number is 0. If it is, it returns 1 (as the factorial of 0 is defined as 1). Otherwise, it recursively multiplies `\$n` with the factorial of `\$n-1` until `\$n` becomes 0, resulting in the factorial of the original number.

The program then prompts the user to enter a number using `readline()`, and checks if it is a positive integer. If it is a negative number, it echoes a message stating that the factorial is not defined. If the number is 0, it directly outputs the factorial as 1. Otherwise, it calls the `factorial()` function with the input number and echoes the result using the `echo` statement.