No database selected PHP [duplicate] - php

Question

This question already has answers here:
Can I mix MySQL APIs in PHP?
(4 answers)
Closed 1 year ago.
I've been trying to spot the error in my database connection , I read several answers suggesting to make sure of the password grant full access ... I checked everything the problem is still appearing ...
My question is ... I understand mysqli is in this case better (from what I read) so ...
if I replace mt $dbcon with
$dbcon = mysqli_connect("$host","$username","$password","$db_name") ;
will that fix the problem and if it does, how do I get rid of the never ending
mysqli_error expects exactly 1 parameter, 0 given ... error
My php is
<?php
$host="****" ;
$username="****" ;
$password="****" ;
$db_name="****" ;
$tbl_name="courses" ;
$dbcon = mysql_connect("$host","$username","$password","$db_name") ;
if (!$dbcon) {
die('error connecting to database'); }
echo 'Courses successfully registerd , ' ;
// escape variables for security
$studentid = ( $_GET['studentid']);
$fname = $_POST["name"]; //echo $studentid;
$query=mysql_query("select * from courses where studentid='".$studentid."' ") or die(mysql_error());
$duplicate=mysql_num_rows($query);
if($duplicate==0)
{
$query1=mysql_query("insert into user values('".$studentid."')") or die(mysql_error());
}
else
{
echo'The ID you entered '.$studentid.' already registered, please wait 24 hours until you can register again';
}
// Get Cources
$name = $_GET['ckb'];
if(isset($_GET['ckb']))
{
foreach ($name as $courcess){
$cc=$cc. $courcess.',';
}
}
$sql="INSERT INTO courses (studentid, ckb, name)
VALUES ('$studentid', '$cc', $fname)";
if (!mysql_query($dbcon,$sql)) {
die('Error: ' . mysqli_error($dbcon));
}
echo " Thank you for using IME Virtual Registeration ";
mysql_close($dbcon);
?>

Answer 1

The problem is in Your PHP on line 7. mysql_connect() has those parameters:
mysql_connect(string $server, string $username, string $password, #optional bool $new_link = false, #optional int $client_flags = 0)
You are giving the fourth parameter $db_name =MISTAKE (fourth parameter must be boolean as you can see.) There's a function mysql_select_db() to tell PHP which database are you using. So here's a rework of your 7th line:
$dbcon = mysql_connect("$host","$username","$password");
mysql_select_db("$db_name");
but I recommend you to use mysqli or PDO ->they are inbuilt PHP libraries, so you don't have to download anything and what's more important they are easy to use.
Good Luck :)

Answer 2

Although we are mostly using a PDO interface now, here is the code also use to connect to the database with some simple command line scripts:
$sql_link=mysql_connect ('host','user','password') or die('Cannot connect to the database because: ' . mysql_error());
if (!$sql_link) {
echo('Could not connect to database : ' . mysql_error());
exit;
}
mysql_select_db('database_name') or die(mysql_error());
Hope that helps!