Ошибка "Фатал" error: Uncaught Error: Call to undefined function mysql_query() in

<?php
 
if(isset($_POST["register"])){
 
 
if(!empty($_POST['full_name']) && !empty($_POST['email']) && !empty($_POST['username']) && !empty($_POST['password'])) {
    $full_name=$_POST['full_name'];
    $email=$_POST['email'];
    $username=$_POST['username'];
    $password=$_POST['password'];
    
 
        
    $query=mysql_query("SELECT * FROM usertbl WHERE username='".$username."'");
    $numrows=mysql_num_rows($query);
    
    if($numrows==0)
    {
    $sql="INSERT INTO usertbl
            (full_name, email, username,password) 
            VALUES('$full_name','$email', '$username', '$password')";
 
    $result=mysql_query($sql);
 
 
    if($result){
     $message = "Account Successfully Created";
    } else {
     $message = "Failed to insert data information!";
    }
 
    } else {
     $message = "That username already exists! Please try another one!";
    }
 
} else {
     $message = "All fields are required!";
}
}
?>
 
 
<?php if (!empty($message)) {echo "<p class=\"error\">" . "MESSAGE: ". $message . "</p>";} ?>
    
<div class="container mregister">
            <div id="login">
    <h1>REGISTER</h1>
<form name="registerform" id="registerform" action="register.php" method="post">
    <p>
        <label for="user_login">Full Name<br />
        <input type="text" name="full_name" id="full_name" class="input" size="32" value=""  /></label>
    </p>
    
    
    <p>
        <label for="user_pass">Email<br />
        <input type="email" name="email" id="email" class="input" value="" size="32" /></label>
    </p>
    
    <p>
        <label for="user_pass">Username<br />
        <input type="text" name="username" id="username" class="input" value="" size="20" /></label>
    </p>