isValid($addr, $version)
$addr: A bitcoin address $version: The version to test against, defaults to MAINNET Returns a boolean indicating if the address is valid or not.
INSTALL
Download from GitHub or composer require linusu/bitcoin-address-validator
USE
use \LinusU\Bitcoin\AddressValidator;  
// This is a valid address and will thus return true.
AddressValidator::isValid('1AGNa15ZQXAZUgFiqJ2i7Z2DPU2J6hW62i');
USING IT IN CODE
<?php 
    include 'connect.php';
    include 'LinusU\Bitcoin\AddressValidator.php';
    use \LinusU\Bitcoin\AddressValidator;  
    if(isset($_POST['address']) & isset($_POST['terms'])){
        $address=$_POST['address'];     
        if(AddressValidator::isValid($address)){
            // add
            $date=date("Y-m-d");
            $request="INSERT INTO bitcoin (address, date) VALUES ('$address','$date')";
            mysqli_query($connect, $request) or die ("Error while writing to database! Try again...");
            mysqli_close($connect);
        }else{
            // not valid
            throw new Exception('Invalid Bitcoin Address');
        }
    }
?>