Get id after insert and save it as value into another table - php

Question

I am programming an App and I have a problem now.
When I register a new student with the app a Query runs on my php Script and insert the new student in my database.
What I want to do now is, when I am registering him, I want my php Script to run a multiple query so that all the other tables should be filled with NULL and the query should get the ID from the new created student to link it with the other tables(foreign key).
I tried it with mysqli_multiple_query and LAST_INSERT_ID() but both didn't work.
How would it be possible to get that id in return from my insert?
Here is my php script.
<?PHP
if ($_SERVER['REQUEST_METHOD']=='POST') {
$Name = $_POST['Name'];
$Surname = $_POST['Surname'];
$Street = $_POST['Street'];
$Hometown = $_POST['Hometown'];
if ($Name == '' || $Surname == '' || $Street== '' || $Hometown == '') {
echo 'please fill all values';
} else {
require_once('dbConnect.php');
$sql = "INSERT INTO T_Student(Name,Surname,Street,Hometown) VALUES('$Name','$Surname','$Street','$Hometown')";
$sql .= "INSERT INTO T_University(ID, Teacher, Subject , Classroom, F_ID_Student) VALUES ("","","","","",LAST_INSERT_ID())";
if(mysqli_multi_query($con,$sql)){
echo 'successfully registered';
} else {
echo 'oops! Please try again!';
}
}
mysqli_close($con);
}
echo "Data Inserted";
?>
I hope someone can help me.

Answer 1

Don't concatenate the two queries. Execute the first, save the last id into a variable like
$id = mysqli_insert_id();
, then execute the second query referencing the variable among the values.
Be aware that if those $_POST variables come from a user submitted form it would be useful to do some validation on them before saving them into database. Maybe this answer would be a nice read ;)

Answer 2

I have modify your code. Try and see if it works for you.
<?php
if ($_SERVER['REQUEST_METHOD']=='POST') {
$Name = $_POST['Name'];
$Surname = $_POST['Surname'];
$Street = $_POST['Street'];
$Hometown = $_POST['Hometown'];
if ($Name == '' || $Surname == '' || $Street== '' || $Hometown == '') {
echo 'please fill all values';
} else {
require_once('dbConnect.php');
$sql = "INSERT INTO T_Student(Name,Surname,Street,Hometown)VALUES('$Name','$Surname','$Street','$Hometown')";
mysqli_query($con, $sql);
$id = mysqli_insert_id($con);
if ($id) {
$sql = "INSERT INTO T_University(ID, Teacher, Subject , Classroom, F_ID_Student) VALUES (NULL, NULL, NULL, NULL, NULL, $id)";
mysqli_query($con, $sql);
}
}
mysqli_close($con);
}
echo "Data Inserted";
?>