How can I troubleshoot a php/mysql insert failure?

How can I troubleshoot a php/mysql insert failure?

I can't get this mysql query to work properly. It completes with no errors
but no information is inserted into the database. I'm not so concerned
about finding an imediate solution but how can I get mysql to report
what's going on behind the scenes?
<?php
error_reporting(E_ALL);
ini_set('display_errors', '1');
$tempProf = $_POST["professor"];
$tempProfArray = explode("=",$tempProf);
$prof = $tempProfArray[1];
$tempName = $_POST["name"];
$tempNameArray = explode("=",$tempName);
$name = $tempNameArray[1];
$tempNum = $_POST["number"];
$tempNumArray = explode("=",$tempNum);
$num = $tempNumArray[1];
$tempSec = $_POST["section"];
$tempSecArray = explode("=",$tempSec);
$section = $tempSecArray[1];
$tempCat = $_POST["category"];
$tempCatArray = explode("=",$tempCat);
$category = $tempCatArray[1];
$con=mysqli_connect("localhost","root","*****","******");
$result = mysqli_query($con,"SELECT * FROM professors where id='$prof'");
$row = mysqli_fetch_array($result);
if(empty($prof) || empty($name) || empty($num) || empty($section) ||
empty($category))
{
echo "emptyField";
}
elseif(!is_numeric($num) || !is_numeric($section))
{
echo "NaN";
}
elseif(empty($row))
{
mysqli_query($con,"INSERT INTO classes (className, classNumber,
section, classCategory)
VALUES ('$name','$num','$section','$category')");
$classTemp = mysqli_query($con,"SELECT id FROM classes where
className='$name' and classNumber='$num' and section ='$section'");
$classTempArray = mysqli_fetch_array($classTemp);
$classId = $classTempArray['id'];
mysqli_query($con,"INSERT INTO professors (name, classes) VALUES
('$name','$classId')");
$profTemp = mysqli_query($con,"SELECT id FROM professors where
name='$name'");
$profTempArray = mysqli_fetch_array($profTemp);
$profId = $classTempArray['id'];
mysqli_query($con,"INSERT INTO classes (professor) VALUES ('$profId')
where id ='$classId'");
}
else
{
$profName = $row['id'];
mysqli_query($con,"INSERT INTO classes ($profName, className,
classNumber, section, classCategory)
VALUES ('$prof', '$name','$num','$section','$category')");
}
?>