Writing to a file in php - php

Question

I am building a website in php. The contact us section has four different fields asking input from user. I want to save the data submitted by the user into a text file names contact.txt. The problem is that the submit button in the form does not work at all.
PHP CODE
<?php
$name=$_POST['name'];
$email=$_POST['email'];
$mob=$_POST['mobile'];
$msg=$_POST['message'];
$txt=" Name=$name \n\r Email=$email \n\r Mobile=$mob \n\r Message=$msg \r\n\r\n\r";
$fh = fopen('contact.txt', 'a+');
fwrite($fh,$txt);
fclose($fh);
?>
HTML CODE
<div id="contact-section">
<h3>Fill the form to Register </h3>
<div class="status alert alert-success" style="display: none"></div>
<form id="main-contact-form" class="contact-form" name="contact-form" method="post" action="sendemail.php">
<div class="form-group">
<input type="text" name="name" class="form-control" required="required" placeholder="Name">
</div>
<div class="form-group">
<input type="text" name="mobile" class="form-control" required="required" placeholder="Mobile">
</div>
<div class="form-group">
<input type="email" name="email" class="form-control" placeholder="Email ID">
</div>
<div class="form-group">
<textarea name="message" id="message" required="required" class="form-control" rows="4" placeholder="Institute/College/School"></textarea>
</div>
<div class="form-group">
<button type="submit" class="btn btn-primary pull-right">Submit</button>
</div>
</form>
</div>
</div
Any help is highly appreciated. Thanks.

Answer 1

Test open success :
if($fh = fopen('contact.txt', 'a+') !== FALSE){
fwrite($fh,$txt);
fclose($fh);
}

Answer 2

Combining my ideas with Fky's here is the most complete code:
<?php
if(!empty($_POST) && array_key_exists('name', $_POST) && array_key_exists('email', $_POST) && array_key_exists('mobile', $_POST) && array_key_exists('message', $_POST)) {
$name=$_POST['name'];
$email=$_POST['email'];
$mob=$_POST['mobile'];
$msg=$_POST['message'];
$txt=" Name=$name \n\r Email=$email \n\r Mobile=$mob \n\r Message=$msg \r\n\r\n\r";
$fh = fopen('contact.txt', 'a');
if($fh = fopen('contact.txt', 'a+') !== FALSE){
fwrite($fh,$txt);
fclose($fh);
}
else {
echo "File can not be opened.";
}
}
else {
echo "Nothing Sent.";
}
?>