The method attribute specifies how to send form-data (the form-data is sent to the page specified in the action attribute).
The form-data can be sent as URL variables (with method="get") or as HTTP post transaction (with method="post").
check here for more details w3schools
<form id="login" class="input-group" method="POST" action="file_path/login.php">
<input type="text" class="input-field" placeholder="User Id" required>
<input type="password" class="input-field" placeholder="Enter Password" required>
<input type="checkbox" class="check-box"><span>Remember Password</span>
<input type="submit" class="submit-btn" value="Sign-In">
</form>
<form Id="register" class="input-group" method="POST" action="file_path/register.php">
<input type="text" class="input-field" placeholder="User Id" required>
<input type="email" class="input-field" placeholder="Email Id" required>
<input type="password" class="input-field" placeholder="Enter Password" required>
<input type="password" class="input-field" placeholder="Confirm Password" required>
<input type="phone-number" class="input-field" placeholder="Mobile Number" required>
<input type="checkbox" class="check-box"><span>I agree to the terms & conditions</span>
<input type="submit" class="submit-btn" value="Sign-Up">
</form>
EDIT according to your comment
replace button with input type="submit"
and place @ before the php variable so you wont get undefined error notice(@ is used to avoid error notice)
<div class="header">
<h2>Register here</h2>
</div>
<form method="post" action="register.php">
<?php include('errors.php'); ?>
<div class="input-group">
<label>Username</label>
<input type="text" name="username" value="<?php echo @$username; ?>">
</div>
<div class="input-group">
<label>Email</label>
<input type="email" name="email" value="<?php echo @$email; ?>">
</div>
<div class="input-group">
<label>Password</label>
<input type="password" name="password_1">
</div>
<div class="input-group">
<label>Confirm Password</label>
<input type="password" name="password_2">
</div>
<div class="input-group">
<label>Mobile number</label>
<input type="number" name="mobile" value="<?php echo @$mobile; ?>">
</div>
<div class="input-group">
<input type="submit" class="btn" name="reg_user" value="Sign-Up">
</div>
</form>
create register.php file
and in register.php
<?php
$con = mysqli_connect("localhost", "username", "password", "dbname") or trigger_error("Unable to connect to the database");
if(isset($_POST['reg_user'])){
$name = $_POST['username']; //here "username" is what you defined in the "name" field of input form
//define other variables
//write your own sql query , here is an example
$query = "INSERT INTO table(name) VALUES(?)";
$stmt = mysqli_stmt_init($con);
if(!mysqli_stmt_prepare($stmt,$query)){
echo "Error";
}else{
mysqli_stmt_bind_param($stmt,"s",$name);
mysqli_stmt_execute($stmt);
}
}
?>