Check for specific entry in database - php

Question

I want to check whether a specific entry is present in my database or not. If present then condition,if not then condition. I tried this code but got errors
<?php
$con=mysqli_connect("localhost","root","","student");
// Check connection
if (mysqli_connect_errno()) {
echo "Failed to connect to MySQL: " . mysqli_connect_error();
}
$classname = mysqli_real_escape_string($con, $_POST['class']);
$result = mysql_query("SELECT * FROM subjectinfo WHERE class = '{$classname}'", $con);
if(mysql_num_rows($result) == 0)
{
echo "No Such Entry In Table. Please ADD it First.";
}
else
{
echo "Entry Available";
}
}
mysqli_close($con);
?>
Errors :
Warning: mysql_query() expects parameter 2 to be resource, object given in C:\xampp\htdocs\pages\test.php on line 11
Warning: mysql_num_rows() expects parameter 1 to be resource, null given in C:\xampp\htdocs\pages\test.php on line 13
No Such Entry In Table. Please ADD it First.

Answer 1

Like your comments. Make sure you don't mix up mysqli and mysql. mysql is deprecated so you're better off using mysqli.
$con=mysqli_connect("localhost","root","","student");
// Check connection
if (mysqli_connect_errno()) {
echo "Failed to connect to MySQL: " . mysqli_connect_error();
}
$classname = mysqli_real_escape_string($con, $_POST['class']);
$result = mysqli_query($con, "SELECT * FROM subjectinfo WHERE class = '{$classname}'");
if(mysqli_num_rows($result) == 0)
{
echo "No Such Entry In Table. Please ADD it First.";
}
else
{
echo "Entry Available";
}
}
mysqli_close($con);
?>

Answer 2

You are mixing MySQL APIs - mysql_ + mysqli_ they do not mix together. Plus, your DB connection's backwards in your query. The connection comes first.
Here:
<?php
$con=mysqli_connect("localhost","root","","student");
// Check connection
if (mysqli_connect_errno()) {
echo "Failed to connect to MySQL: " . mysqli_connect_error();
}
$classname = mysqli_real_escape_string($con, $_POST['class']);
$result = mysqli_query($con,"SELECT * FROM subjectinfo WHERE class = '{$classname}'");
if(mysqli_num_rows($result) == 0)
{
echo "No Such Entry In Table. Please ADD it First.";
}
else
{
echo "Entry Available";
}
}
mysqli_close($con);
?>
Also use or die(mysqli_error($con)) to mysqli_query()
Plus, add error reporting to the top of your file(s) which will help during production testing.
error_reporting(E_ALL);
ini_set('display_errors', 1);
Look into using prepared statements, or PDO with prepared statements, they're safer.
An insight:
Make sure your form element is indeed named.
I.e.:
<input type="text" name="class">
otherwise, you will receive an Undefined index class... warning.