PHP write to text file from input

I have the following code

                                <div id="mailbox">
                                    <form action="verification.php" method="post">
                                    <input placeholder="Enter your email" type="mail" name="name" style="width:270px; height:42px; border: solid 1px #c2c4c6; font-size:16px; padding-left:8px;" />
                            </div>
                            <div>
                               <form action="verification.php" method="post">
                               <input type="submit" id="button2" value="Next" />

I want if a user put an input in the enter your email placeholder - it will save the input into a text file.

I have no idea what code I need to write in verification.php. I have 2 days knowledge in PHP.

I want every input a user put in the form, will be saved to user.txt file on my machine.

Thank you