PHP email glitch - php

Question

This is a school assignment. I don't know all of the fine details of PHP variables and such, but! If I hardcode the id then it sends just like it should. If not, it doesn't at all. Suggestions?
if ((!empty($_subject)) && (!empty($_text))) {
$_dbc = mysqli_connect('localhost', 'user', 'password', 'db') or die ('Error Connecting to MySQL server.');
$_id = $_GET['id'];
$_query = "SELECT * FROM midterm WHERE id = '$_id'";
$_result = mysqli_query($_dbc, $_query) or die ('Error Querying Database.');
while($_row = mysqli_fetch_array($_result)) {
$_to = $_row['email'];
$_firstName = $_row['firstName'];
$_msg = "Dear $_firstName, /n $_text";
mail($_to, $_subject, $_msg, 'From:' . $_from);
echo 'Mail sent to: ' . $_firstName . ' at ' . $_to . '<br />';
}

Answer 1

Addressing $_GET/$_POST problems with a NULL $_GET['id']
<form name="example" action="http://example.com" method="POST"></form>
OR
<form name="example" action="http://example.com" method="GET"></form>
If variables are POSTed from a form, they will be sent through the server and you can access them on the submitted page with $_POST['key']. If variables are sent with GET, they will be appended to the URL in a query string and accessible on the page with $_GET['key'].
When you use the variable $_GET['id'] it is looking in the URL's query string for the key id. If your URL looked like http://example.com/sript.php?id=123 then $_GET['id'] would return 123 not NULL. This can either be accomplished by visiting a specific URL (like shown above) or arriving on the page through a form like below:
<form name="example" action="http://example.com/script.php" method="GET">
<input type="text" name="id" />
<input type="submit" value="Try Me" />
</form>
This form, when submitted, will send the $_GET['id'] variable to script.php using GET.
Addressing other issues with mail not sending
Can you add the following lines of code before your while loop and let me know what they say? This will let me know if your MySQL query is working.
echo "Rows returned: ".mysqli_num_rows($_result)."<br />";
// this line only works on PHP > 5.3.0
echo "Result dump:<br /><pre>".mysqli_fetch_all($_result)."</pre>";
Since mysqli_num_rows() returned 0, that means that your SQL query SELECT * FROM midterm WHERE id = '7' is not returning any results. I can't help any more without looking at the database structure. Double check your code (and compare it to code that works) to try to see what is going wrong. Make sure that there is a row in the table midterm that has an id = 7.