INSERT query in PHP not working - php

Question

I am really new to PHP and am messing around with adding and editing database values. I have accomplished adding information to the database, but I cannot seem to figure out how to edit it properly!
I created a file called edit.php and here is the code I have thus far:
<?php
include '../database/connection.php';
$id = $_GET['id'];
$first_name = $_GET['first_name'];
$last_name = $_GET['last_name'];
$university_id = $_GET['university_id'];
$sql = "UPDATE master_roster (first_name, last_name, university_id) VALUES ('$first_name', '$last_name', '$university_id') WHERE id = $id";
?>
No error messages of any help are posting. Whenever the form is submitted and is handed off to this file, I just get a blank screen with no results. I cannot seem to figure out what it is I am missing to have it update the content from input fields!
EDIT: I gave all of suggestions a shot but it still does not work! Here is the form that the data is coming from:
<?php
$id = $_GET['id'];
$first_name = $_GET['first_name'];
$last_name = $_GET['last_name'];
$university_id = $_GET['university_id'];
?>
<form action="edit_member.php?id=<?php echo $id; echo "&first_name="; echo $first_name; echo "&last_name="; echo $last_name; echo "&university_id="; echo $university_id; ?>" method="post">
<table>
<tr>
<td>First Name</td>
<td><input type="text" name="first_name" value="<?php echo $first_name; ?>"></td>
</tr>
<tr>
<td>Last Name</td>
<td><input type="text" name="last_name" value="<?php echo $last_name; ?>"></td>
</tr>
<tr>
<td>University ID</td>
<td><input type="text" name="university_id" value="<?php echo $university_id; ?>"></td>
</tr>
<tr>
<td></td>
<td><input type="submit" value="Submit"></td>
</tr>
</table>
</form>
I am not too concerned about SQL Injections at this point because I am just trying to learn the basics.
EDIT #2:
LIST.PHP - where the DB pulls all the members
<table>
<tr>
<th>ID</th>
<th>First Name</th>
<th>Last Name</th>
<th>University ID</th>
</tr>
<?php
include '../database/connection.php';
$sql = "SELECT * FROM master_roster";
if($result = mysqli_query($link, $sql)){
if(mysqli_num_rows($result) > 0) {
while($row = mysqli_fetch_array($result)){
echo "<tr>";
echo "<td>" . $row['id'] . "</td>";
echo "<td>" . $row['first_name'] . "</td>";
echo "<td>" . $row['last_name'] . "</td>";
echo "<td>" . $row['university_id'] . "</td>";
echo "<td><a href='form.php?id=" . $row['id'] . "&first_name=" . $row['first_name'] . "&last_name=" . $row['last_name'] . "&university_id=" . $row['university_id'] . "'>Edit</a></td>";
echo "</tr>";
}
echo "</table>";
mysqli_free_result($result);
} else {
echo "No records matching your query were found.";
}
} else{
echo "ERROR: Could not able to execute $sql. " . mysqli_error($link);
}
mysqli_close($link);
?>
EDIT.PHP
<?php
include '../database/connection.php';
$id = mysqli_real_escape_string($_POST['id']);
$first_name = mysqli_real_escape_string($_POST['first_name']);
$last_name = mysqli_real_escape_string($_POST['last_name']);
$university_id = mysqli_real_escape_string($_POST['university_id']);
$sql = "UPDATE master_roster
SET
first_name = '$first_name',
last_name = '$last_name',
university_id = '$university_id'
WHERE
id = $id";
?>
FORM.PHP
<form action="edit.php" method="post">
<table>
<tr>
<td>First Name</td>
<td><input type="text" name="first_name"></td>
</tr>
<tr>
<td>Last Name</td>
<td><input type="text" name="last_name"></td>
</tr>
<tr>
<td>University ID</td>
<td><input type="text" name="university_id"></td>
</tr>
<tr>
<td><input type="submit" value="Submit"></td>
</tr>
</table>
</form>

Answer 1

This is the wrong syntax for update. update takes a series of column=value clauses:
UPDATE master_roster
SET first_name = '$first_name',
last_name = '$last_name',
university_id = '$university_id'
WHERE id = $id
Mandatory comment:
Using variable substitutions in strings like that leaves your code vulnerable to SQL injection attacks. You should consider using a prepared statement instead.

Answer 2

You're writing wrong UPDATE statement.
Try this
$sql = "UPDATE master_roster
SET
first_name = '$first_name',
last_name = '$last_name',
university_id = '$university_id'
WHERE
id = $id";
Now execute the query using statement below.
$result = $conn->query($sql);
$result will return true if the insertion is successful and false if not.
Use this to check if it is done or not.
if($result == false){
die( "Connection Failed: ".$conn->error );
}
You should also be prevented from SQL injection.
You can use mysqli_real_escape_string() method.
$first_name = $_GET['first_name'];
$safe_first_name = mysqli_real_escape_string($conn, $first_name);
You can also use parameterized query.
Read this.
Now read the codes below modify your both pages like this.
Form
<form action="edit.php" method="post">
<table>
<tr>
<td>First Name</td>
<td><input type="text" name="first_name"></td>
</tr>
<tr>
<td>Last Name</td>
<td><input type="text" name="last_name"></td>
</tr>
<tr>
<td>University ID</td>
<td><input type="text" name="university_id"></td>
</tr>
<tr>
<td><input type="submit" value="Submit"></td>
</tr>
</table>
</form>
This will update the data.
<?php
include '../database/connection.php';
$id = mysqli_real_escape_string($conn, $_POST['id']);
$first_name = mysqli_real_escape_string($conn, $_POST['first_name']);
$last_name = mysqli_real_escape_string($conn, $_POST['last_name']);
$university_id = mysqli_real_escape_string($conn, $_POST['university_id']);
$sql = "UPDATE master_roster
SET
first_name = '$first_name',
last_name = '$last_name',
university_id = '$university_id'
WHERE
id = $id";
$result = $conn->query($sql);
if($result == false){
die( "Connection Failed: ".$conn->error );
}
?>