Unable to echo value from html <option> from while loop - php

Question

i want to echo selected parent value. but i am getting error- Notice: Undefined index:
How can i echo selected parent value then? Whats wrong i am doing?
$q = mysql_query("SELECT * FROM menu");
echo '<form action="" method="post">
Menu name:<input type="text" name="mname"><br>
<select>';
while ($row = mysql_fetch_array($q)) {
$menu_name = $row['menu_name'];
echo '<option value="'.$menu_name.'">'.$menu_name.'</option>';
}
echo '</select><br>
<input type="submit" name="submit" value="Add Menu">
</form>';
if (isset($_POST['submit'])) {
echo $mname = $_POST['mname'];
echo $parent = $_POST[$menu_name];
}

Answer 1

add name to the select box and get the value of select box by name.
Updated code:-
$q = mysql_query("SELECT * FROM menu");
echo '<form action="" method="post">
Menu name:<input type="text" name="mname"><br>
<select name="menu_name">';
while ($row = mysql_fetch_array($q)) {
$menu_name = $row['menu_name'];
echo '<option value="'.$menu_name.'">'.$menu_name.'</option>';
}
echo '</select><br>
<input type="submit" name="submit" value="Add Menu">
</form>';
if (isset($_POST['submit'])) {
echo $mname = $_POST['mname'];
echo $parent = $_POST['menu_name'];
}

Answer 2

$_POST[$menu_name] probably doesn't exist, because only two elements in your form have name attributes. The text input and the submit input.
option elements aren't posted as part of the form, but rather the selected option's value for the select element. But your select element has no name, therefore no key to use in the key/value pair, so it isn't posted.
Give the element a name:
<select name="someName">
Then in the POST, you would be able to fetch the selected value just as you do for any other form element:
$_POST['someName']

Answer 3

You need to add name attribute to select tag.
echo '<form action="" method="post">
Menu name:<input type="text" name="mname"><br>
<select name="any_name">';
$q = mysql_query("SELECT * FROM menu");
while ($row = mysql_fetch_array($q)) {
$menu_name = $row['menu_name'];
echo '<option value="'.$menu_name.'">'.$menu_name.'</option>';
}
echo '</select><br>
<input type="submit" name="submit" value="Add Menu">
</form>';
if (isset($_POST['submit'])) {
echo $mname = $_POST['mname'];
echo $select_option_name = $_POST['any_name'];
}
Note: mysql_* functions are depricated, use mysqli_* functions