The most common function used in Ethereum application development is the Keccak256 hashing. Keccak256 is used for Ethereum message signing, obtaining the wallet address given a public key, determining smart contract address and determining the function selector of smart contract to be provided as data in transactions given a function prototype (like in ABI encoding). I have looked around and could not find a useful implementation for this. They are either unfinished like this one or too complicated to use like this one, or this one.
All I need is a simple keccak256() function that returns keccak-256 of a hex encoded string. So I decided to do it for myself. I looked around a bit and found a clear C program in this page. All I need is to make this code work in PHP. Luckily PHP has a mechanism to extent its functionality. I followed the guide here. Then I obtained a working keccak256 function.
In this github page, I published the codes to be used by anyone interested.
Before installation, PHP developer files must be installed. To install keccak256() functionality:

• git clone https://github.com/RnDevelover/Keccak256PHP.git
• cd Keccak256PHP
• phpize
• ./configure --enable-keccak256
• make
• copy modules/keccak256.so to your php extension directory.
• enable extension by adding extension=keccak256.so to your php.ini

Usage:

$a="cc"; // Hex encoded string. All characters are [0-9a-fA-F].
$hash=keccak256($a);
echo $hash; // Hex encoded hash.

This function returns a string of hex encoded 256 bit data (64 characters where each two character indicates one byte).