PHP

<?php
$host = "localhost";
$user = "root";
$pass = "";
$conn = mysqli_connect($host, $user, $pass);
echo "Connected successfully<br>";
echo "Host info: " . mysqli_get_host_info($conn) . "<br>";
echo "Server version: " . mysqli_get_server_info($conn);
mysqli_close($conn);
?>

<?php
$conn = mysqli_connect("localhost", "root", "wrongpass")
    or die("Could not connect: " . mysqli_connect_error());
echo "Connected";
?>

<?php
try {
    $conn = mysqli_connect("localhost", "root", "wrongpass", "phpbook");
    echo "Connected";
} catch (mysqli_sql_exception $e) {
    error_log($e->getMessage());                       // details for the developer
    echo "Sorry, the database is not available right now.<br>";
    echo "(Error " . $e->getCode() . ")";
}
?>

<?php
$conn = mysqli_connect("localhost", "root", "", "phpbook");
mysqli_set_charset($conn, "utf8mb4");

<?php
$conn = mysqli_connect("localhost", "root", "");
mysqli_select_db($conn, "phpbook");
$r = mysqli_query($conn, "SELECT DATABASE() AS db");
$row = mysqli_fetch_assoc($r);
echo "Database " . $row['db'] . " selected";
mysqli_close($conn);
?>

<?php
require_once "db.php";
mysqli_query($conn, "DROP TABLE IF EXISTS enquiry");
$sql = "CREATE TABLE IF NOT EXISTS enquiry (
          id INT AUTO_INCREMENT PRIMARY KEY,
          name VARCHAR(50) NOT NULL,
          mobile VARCHAR(10),
          course VARCHAR(30),
          enquired_on DATETIME DEFAULT CURRENT_TIMESTAMP)";
if (mysqli_query($conn, $sql)) {
    echo "Table enquiry created";
}
mysqli_close($conn);
?>

<form action="insert.php" method="post" style="font-family:Arial;font-size:13px">
  Name: <input name="name" id="name"><br>
  Mobile: <input name="mobile" id="mobile"><br>
  Course: <select name="course" id="course">
            <option>Core PHP</option><option>Advance PHP</option><option>Laravel</option>
          </select><br>
  <input type="submit" value="Save" id="save">
</form>

<?php
require_once "db.php";
$name   = mysqli_real_escape_string($conn, $_POST['name']);
$mobile = mysqli_real_escape_string($conn, $_POST['mobile']);
$course = mysqli_real_escape_string($conn, $_POST['course']);
$sql = "INSERT INTO enquiry (name, mobile, course)
        VALUES ('$name', '$mobile', '$course')";
if (mysqli_query($conn, $sql)) {
    echo "Record saved. Enquiry number: " . mysqli_insert_id($conn);
}
mysqli_close($conn);
?>

<?php
require_once "db.php";
$result = mysqli_query($conn, "SELECT roll, name, city FROM students ORDER BY roll");
echo "Total students: " . mysqli_num_rows($result) . "<br>";
echo "<table border='1' cellpadding='4' style='border-collapse:collapse;font-family:Arial;font-size:13px'>";
echo "<tr><th>Roll</th><th>Name</th><th>City</th></tr>";
while ($row = mysqli_fetch_array($result)) {
    echo "<tr><td>" . $row['roll'] . "</td><td>" . $row['name']
       . "</td><td>" . $row[2] . "</td></tr>";
}
echo "</table>";
mysqli_free_result($result);
mysqli_close($conn);
?>

<?php
require_once "db.php";
mysqli_query($conn, "UPDATE students SET city = 'Sehore' WHERE roll = 103");
echo mysqli_affected_rows($conn) . " record(s) updated<br>";
$r = mysqli_query($conn, "SELECT name, city FROM students WHERE roll = 103");
$row = mysqli_fetch_assoc($r);
echo "Now: {$row['name']} lives in {$row['city']}";
mysqli_close($conn);
?>

<?php
require_once "db.php";
$roll = (int) $_GET['roll'];               // cast to int – only a number can reach the SQL
mysqli_query($conn, "DELETE FROM enquiry WHERE id = $roll");
echo mysqli_affected_rows($conn) . " record(s) deleted<br>";
$r = mysqli_query($conn, "SELECT id, name FROM enquiry");
while ($row = mysqli_fetch_row($r)) {
    echo "Remaining: #$row[0] $row[1]<br>";
}
mysqli_close($conn);
?>

<?php
require_once "db.php";
$city = mysqli_real_escape_string($conn, $_GET['city'] ?? '');
?>
<form style="font-family:Arial;font-size:13px">
  City: <input name="city" value="<?= htmlspecialchars($city) ?>"> <input type="submit" value="Search">
</form>
<?php
$result = mysqli_query($conn, "SELECT name, city FROM students WHERE city LIKE '%$city%'");
if (mysqli_num_rows($result) == 0) {
    echo "No student found";
}
while ($row = mysqli_fetch_assoc($result)) {
    echo htmlspecialchars($row['name']) . " – " . htmlspecialchars($row['city']) . "<br>";
}
mysqli_close($conn);
?>

<?php
require_once "db.php";
$sql = "SELECT roll, name FROM students ORDER BY roll LIMIT 3";
echo "<pre>";
$r = mysqli_query($conn, $sql);
echo "mysqli_fetch_row:    "; print_r(mysqli_fetch_row($r));
echo "mysqli_fetch_assoc:  "; print_r(mysqli_fetch_assoc($r));
$obj = mysqli_fetch_object($r);
echo "mysqli_fetch_object: roll = $obj->roll, name = $obj->name\n\n";
$r = mysqli_query($conn, $sql);
echo "mysqli_fetch_array(MYSQLI_BOTH): "; print_r(mysqli_fetch_array($r, MYSQLI_BOTH));
$r = mysqli_query($conn, $sql);
$all = mysqli_fetch_all($r, MYSQLI_ASSOC);
echo "mysqli_fetch_all: " . count($all) . " rows, last = " . $all[2]['name'] . "\n";
echo "Columns: ";
foreach (mysqli_fetch_fields($r) as $f) echo $f->name . " ";
echo "</pre>";
mysqli_close($conn);
?>

<?php
require_once "db.php";
$sql = "SELECT s.roll, s.name, c.course_name, c.fee
        FROM students s
        LEFT JOIN courses c ON s.course_id = c.course_id
        ORDER BY s.roll";
$result = mysqli_query($conn, $sql);
echo "<table border='1' cellpadding='4' style='border-collapse:collapse;font-family:Arial;font-size:13px'>";
echo "<tr><th>Roll</th><th>Name</th><th>Course</th><th>Fee (Rs.)</th></tr>";
$total = 0;
while ($row = mysqli_fetch_assoc($result)) {
    $total += $row['fee'];
    echo "<tr><td>{$row['roll']}</td><td>{$row['name']}</td>"
       . "<td>{$row['course_name']}</td><td align='right'>" . number_format($row['fee'], 2) . "</td></tr>";
}
echo "<tr><th colspan='3' align='right'>Total</th><th align='right'>" . number_format($total, 2) . "</th></tr></table>";
mysqli_close($conn);
?>

<?php
require_once "db.php";
try {
    mysqli_query($conn, "SELECT * FROM student");          // wrong table name
} catch (mysqli_sql_exception $e) {
    echo "Exception " . $e->getCode() . ": " . $e->getMessage() . "<br>";
}
mysqli_report(MYSQLI_REPORT_OFF);                         // old style: no exceptions
$r = mysqli_query($conn, "SELEC name FROM students");      // syntax error
if (!$r) {
    echo "mysqli_errno: " . mysqli_errno($conn) . "<br>";
    echo "mysqli_error: " . mysqli_error($conn);
}
mysqli_close($conn);
?>

<?php
require_once "db.php";
$sql = "SELECT COUNT(*) FROM students;
        SELECT COUNT(*) FROM courses;
        SELECT MAX(fee) FROM courses";
$labels = ["Students", "Courses", "Highest fee"];
$i = 0;
if (mysqli_multi_query($conn, $sql)) {
    do {
        if ($result = mysqli_store_result($conn)) {
            $row = mysqli_fetch_row($result);
            echo $labels[$i++] . ": " . $row[0] . "<br>";
            mysqli_free_result($result);
        }
    } while (mysqli_next_result($conn));
}
mysqli_close($conn);
?>

<?php
require_once "db.php";
$msg = "";
$edit = ["course_id" => "", "course_name" => "", "duration" => "", "fee" => ""];

// CREATE or UPDATE
if ($_SERVER["REQUEST_METHOD"] == "POST") {
    $id   = (int) ($_POST["course_id"] ?? 0);
    $name = mysqli_real_escape_string($conn, trim($_POST["course_name"]));
    $dur  = mysqli_real_escape_string($conn, trim($_POST["duration"]));
    $fee  = (float) $_POST["fee"];
    if ($id > 0) {
        mysqli_query($conn, "UPDATE courses SET course_name='$name', duration='$dur', fee=$fee
                             WHERE course_id=$id");
        $msg = "Course updated (" . mysqli_affected_rows($conn) . " row).";
    } else {
        mysqli_query($conn, "INSERT INTO courses (course_name, duration, fee)
                             VALUES ('$name', '$dur', $fee)");
        $msg = "Course added with id " . mysqli_insert_id($conn) . ".";
    }
}
// DELETE
if (isset($_GET["delete"])) {
    $id = (int) $_GET["delete"];
    mysqli_query($conn, "DELETE FROM courses WHERE course_id=$id");
    $msg = "Course deleted.";
}
// load a course into the form for editing
if (isset($_GET["edit"])) {
    $id = (int) $_GET["edit"];
    $r = mysqli_query($conn, "SELECT * FROM courses WHERE course_id=$id");
    $edit = mysqli_fetch_assoc($r) ?? $edit;
}
// READ
$result = mysqli_query($conn, "SELECT * FROM courses ORDER BY course_id");
?>
<html><body style="font-family:Arial;font-size:13px">
<h3>Courses</h3>
<p style="color:green"><?= $msg ?></p>
<form method="post" action="courses.php">
  <input type="hidden" name="course_id" value="<?= $edit['course_id'] ?>">
  Name <input id="course_name" name="course_name" size="12" value="<?= htmlspecialchars($edit['course_name']) ?>">
  Duration <input id="duration" name="duration" size="8" value="<?= htmlspecialchars($edit['duration']) ?>">
  Fee <input id="fee" name="fee" size="6" value="<?= $edit['fee'] ?>">
  <input type="submit" id="save" value="<?= $edit['course_id'] ? 'Update' : 'Add' ?>">
</form><br>
<table border="1" cellpadding="4" style="border-collapse:collapse">
<tr><th>Id</th><th>Course</th><th>Duration</th><th>Fee</th><th>Action</th></tr>
<?php while ($row = mysqli_fetch_assoc($result)): ?>
<tr>
  <td><?= $row['course_id'] ?></td>
  <td><?= htmlspecialchars($row['course_name']) ?></td>
  <td><?= htmlspecialchars($row['duration']) ?></td>
  <td align="right"><?= $row['fee'] ?></td>
  <td><a href="courses.php?edit=<?= $row['course_id'] ?>">Edit</a> |
      <a href="courses.php?delete=<?= $row['course_id'] ?>">Delete</a></td>
</tr>
<?php endwhile; ?>
</table>
</body></html>
<?php mysqli_close($conn); ?>