<?php
if(isset($_POST['submit'])){
    $input = $_POST['input'];
    if(preg_match('#^https?://#i', $input)){
        header("Location: https://vir9.com");
        exit();
    }
}
?>

another ex

<?php
if($_POST['submit-board']) {
    $url=$_POST['url'];	
    $thai=$_POST['thai'];	
    $topic=$_POST['topic'];
    $detail=$_POST['detail'];			
    $tel=$_POST['tel'];
    $email=$_POST['email'];
    $ip = isset($_SERVER['REMOTE_ADDR']) ? $_SERVER['REMOTE_ADDR'] : '';

    // check if URL is entered and redirect to cymiz.com if it is
    if(preg_match('#^https?://#i', $url)){
        header("Location: https://vir9.com");
        exit();
    } else {
        // send the email if the URL is not redirected
        $to = "xxx@vir9.com";
        $subject = "vir9.com $thai"; //1
        $sub="..."; //2
        
        // send the email using the mail() function
        mail($to,$subject,$sub,"From: $email");
    }
}
?>