form save empty value php - php

Question

I have created a profile page in php, where the user, using a form can save his telephone number in a table named profile in database. If the user decides to change his telephone number can easily go to form change it and save it again. My only problem is the following. Suppose that the user wants to delete his telephone number. If he goes to the form delete his number (makes the field empty) and press save , the telephone number doesn't change to an empty value and continues to keep the previous number. Any idea how to change this in order to keep an empty value?
This is my code with the form :
<form action="" method="POST" >
<?php
if ( isset($_GET['success']) === true && empty($_GET['success'])===true ){
echo'Profile Updated Sucessfuly';
}else{
if( empty($_POST) === false && empty($errors) === true ){
$update_data_profile = array('telephone' => $_POST['telephone']);
update_user_profile($session_user_id, $update_data_profile);
header('Location: profile_update.php?success');
exit();
}else if ( empty($errors) === false ){
echo output_errors($errors);
}
?>
Telephone<input name="telephone" type="text" size="25" value="<?php echo $user_data_profile['telephone']; ?>"/>
<input type="submit" value="" name="submit"/>
</form>
And this is my function that dispatches data to profile table:
function update_user_profile($user_id, $update_data_profile){
$result = mysql_query("select user_id from profile where user_id = $user_id limit 1");
if(mysql_num_rows($result) === 1){
$update = array();
array_walk($update_data_profile, 'array_sanitize');
foreach($update_data_profile as $field => $data ){
if(!empty($data)){
$update[]='`' . $field . '` = \'' . $data . '\'';
}
}
if(isset($update) && !empty($update)){
mysql_query(" UPDATE `profile` SET " . implode(', ', $update) . " WHERE `user_id` = $user_id ") or die(mysql_error());
}
}
else{
$user_id = $update_data_profile['user_id'] ;
if(count($update_data_profile)){
$columns = array();
$values = array();
foreach($update_data_profile as $field => $data){
$columns[] = $field;
$values[] = $data;
}
}
mysql_query(" INSERT INTO `profile` (" . implode(",", $columns) .") values ('" . implode("','", $values) . "')" ) or die (mysql_error());
}
}

Answer 1

You are only updating the field if the data is NOT empty.
See this line:
if(!empty($data)){

Answer 2

Because you're explicitly ignoring empty values with this:
if(!empty($data))

Answer 3

Since empty() function can validate for missing, and false elements, you can't simply pass that on to a SQL query. So, if you want to actually generate a query for these items, you will need to set the value explicitly by doing something like this:
if (empty($data)) {
$update[] = "`{$field}` = ''" ;
} else {
$update[] = "`{$field}` = '{$data}'" ;
}
If you are writing for PHP 5.3 or above, you could also replace the if..else statement and shorten your code by using a ternary operator (conditional assignment) doing something like this:
$update[] = (empty($data)) ? "`{$field}` = ''" : "`{$field}` = '{$data}'";