The mysqli_query() function in PHP is used to execute a query against a MySQL database using the MySQLi extension.

🔹 Syntax

mysqli_query(mysqli $connection, string $query, int $resultmode = MYSQLI_STORE_RESULT): mysqli_result|bool

• $connection: A valid connection object (from mysqli_connect()).
• $query: The SQL query to execute (e.g., SELECT, INSERT, UPDATE, DELETE).
• $resultmode: Optional. Used for SELECT queries to specify how results are returned (default is MYSQLI_STORE_RESULT).

🔹 Return Values

• SELECT queries return a mysqli_result object on success, or false on failure.
• Other queries (INSERT, UPDATE, DELETE, etc.) return true on success, or false on failure.

🔹 Examples

✅ Example 1: SELECT Query

<?php
$conn = mysqli_connect("localhost", "root", "", "testdb");

$sql = "SELECT id, name FROM users";
$result = mysqli_query($conn, $sql);

if ($result) {
    while ($row = mysqli_fetch_assoc($result)) {
        echo "ID: " . $row["id"] . " - Name: " . $row["name"] . "<br>";
    }
} else {
    echo "Error: " . mysqli_error($conn);
}

mysqli_close($conn);
?>

✅ Example 2: INSERT Query

<?php
$conn = mysqli_connect("localhost", "root", "", "testdb");

$sql = "INSERT INTO users (name, email) VALUES ('John Doe', 'john@example.com')";
if (mysqli_query($conn, $sql)) {
    echo "New record created successfully";
} else {
    echo "Error: " . mysqli_error($conn);
}

mysqli_close($conn);
?>

✅ Example 3: UPDATE Query

<?php
$conn = mysqli_connect("localhost", "root", "", "testdb");

$sql = "UPDATE users SET name = 'Jane Doe' WHERE id = 1";
if (mysqli_query($conn, $sql)) {
    echo "Record updated successfully";
} else {
    echo "Error: " . mysqli_error($conn);
}

mysqli_close($conn);
?>

✅ Example 4: DELETE Query

<?php
$conn = mysqli_connect("localhost", "root", "", "testdb");

$sql = "DELETE FROM users WHERE id = 1";
if (mysqli_query($conn, $sql)) {
    echo "Record deleted successfully";
} else {
    echo "Error: " . mysqli_error($conn);
}

mysqli_close($conn);
?>

⚠️ Notes:

• Always sanitize user input to prevent SQL Injection.
• Use prepared statements (mysqli_prepare) for safer queries involving user input.