DELETE data from database if it is exists - php

Question

I want to delete data from database if the same exists in database, but my coding is not working. if user input data is not matching then a error warning should be display.
HTML
<form action="" method="POST">
<input type="number" name="student_id_delete" placeholder="Enter Student ID"/>
<input type="submit" name="sub_delete" value="Delete"/>
</form>
PHP
define('HOST','localhost');
define('USER','root');
define('PASSWORD_HOST','');
define('DATABASE','ubhs');
if(defined('HOST') && defined('USER') && defined('PASSWORD_HOST') && defined('DATABASE')){
$conn = mysqli_connect(HOST, USER, PASSWORD_HOST, DATABASE);
}else{
die(connection_failed.mysqli_connection_error());
}
$userinput = true;
$student_id_delete = $_POST['student_id_delete'];
if(isset($_POST['sub_delete'])){
if(empty($student_id_delete)){
$userErr1 = "Please enter student ID to be deleted";
$userinput = false;
}
$sql = "SELECT FROM student_info WHERE id=$student_id_delete";
$result = mysqli_query($conn, $sql);
if (mysqli_num_rows($result)<0){
echo "student could not be found in database";
$userinput = false;
}
if($userinput==true){
$sql = "DELETE FROM student_info WHERE id=$student_id_delete";
if(mysqli_query($conn,$sql)){
echo "Success";
}else{
echo "Couldn't deleted";
}
}
}

Answer 1

Try below snippet :
HTML code :
<form action="" method="POST">
<input type="number" name="student_id_delete" placeholder="Enter Student ID"/>
<input type="submit" name="sub_delete" value="Delete"/>
</form>
PHP code :
<?php
define('HOST','localhost');
define('USER','root');
define('PASSWORD_HOST','');
define('DATABASE','ubhs');
if(defined('HOST') && defined('USER') && defined('PASSWORD_HOST') && defined('DATABASE'))
{
$conn = mysqli_connect(HOST, USER, PASSWORD_HOST, DATABASE);
}
else
{
die(connection_failed.mysqli_connection_error());
}
$userinput = true;
$student_id_delete = $_POST['student_id_delete'];
if(isset($_POST['sub_delete']))
{
if(empty($student_id_delete))
{
$userErr1 = "Please enter student ID to be deleted";
$userinput = false;
}
if($userinput==true)
{
$sql = "DELETE FROM student_info WHERE id=".mysqli_real_escape_string($student_id_delete); // sql injection prevention.
mysqli_query($conn,$sql);
echo "Records deleted: ".mysqli_affected_rows($conn);
}
}
?>
OUTPUT :
Records deleted: 1
Records deleted: 0