Form validation in PHP

<!DOCTYPE html>
<html>
<head>
    <title>Small PHP Form Validation</title>
    <style>
        .error{color:red}
    </style>
</head>
<body>
<?php
if(isset($_POST['form1'])) {
    $u_name = $_POST['u_name'];
    $u_email = $_POST['u_email'];
    $u_username = $_POST['u_username'];
    $u_password = $_POST['u_password'];

    $valid=1;

    if(empty($u_name)) {
        echo "<div class='error'>Your name Can't be Empty</div><br>";
        $valid = 0;
    }
    if(empty($u_email)) {
        echo "<div class='error'>Your Email Can not be Empty</div><br>";
        $valid = 0;
    }
    if(empty($u_username)) {
        echo "<div class='error'>Username Can not be Empty</div><br>";
        $valid = 0;
    }
    if(empty($u_password)) {
        echo "<div class='error'>Password Can not be Empty</div><br>";
        $valid = 0;
    }

    if(!(preg_match("/^[_a-z0-9-]+(\.[_a-z0-9-]+)*@[a-z0-9-]+(\.[a-z0-9-]+)*(\.[a-z]{2,3})$/", $u_email))) {
        echo "<div class='error'>Email Not Valid. Please Enter Valid Email Address</div>";
        $valid = 0;
    }
    if($valid == 1) {
        echo "Success";
    }
    else {
        echo "Wrong";
    }
}
else{?>
<form action="" method="post">
    <table>
        <tr>
            <td>Name: </td>
            <td><input type="text" name="u_name" placeholder="Full Name"  /></td>
        </tr>
        <tr>
            <td>Email Address: </td>
            <td><input type="text" name="u_email" placeholder="Email Address" ></td>
        </tr>
        <tr>
            <td>Username: </td>
            <td><input type="text" name="u_username" placeholder="Username" /></td>
        </tr>
        <tr>
            <td>Password: </td>
            <td><input type="password" name="u_password" placeholder="Password" /></td>
        </tr>
        <tr>
            <td><input type="submit" value="SEND" name="form1"/></td>
        </tr>
    </table>
    <body>
</html>

<?php
}

?>