php selected option Undefined index

Question

I get Undefined index on the 6th line (if condition). What is the reason?

<select name="day" id="day">
<?php
for($i=01; $i<=31; $i++)
{
    echo "<option value='$i'";
    if ($_POST['day'] == $i){ echo "selected";}
    echo">$i</option>";
}
 ?>
</select>

Answer 1

That's beacuse $_POST['day'] is not set.

Answer 2

The message is crystal clear: the key day does not (always) exist in $_POST.

So you have to check if it does first:

if (isset($_POST['day']) && ($_POST['day']==$i)) { 
    echo "selected";
}