The mysqli_query() function in PHP is used to execute SQL queries against a MySQL database when using the MySQLi extension in procedural style. Here's a detailed explanation:

Syntax

mysqli_query(connection, query, resultmode)

Parameters

1. connection: The MySQLi connection object returned by mysqli_connect(). This parameter specifies the connection to the database that will be used for the query.

2. query: The SQL query string to be executed. This can be any valid SQL statement, such as SELECT, INSERT, UPDATE, or DELETE.

3. resultmode (optional): This parameter specifies the type of result to return. The possible values are:

   • MYSQLI_STORE_RESULT (default): The result set is stored in memory.
   • MYSQLI_USE_RESULT: The result set is fetched directly from the database server, which can be more efficient for large result sets.

Return Values

• On success:
  • For SELECT queries, mysqli_query() returns a mysqli_result object, which you can use to fetch rows from the result set.
  • For INSERT, UPDATE, or DELETE queries, mysqli_query() returns true on success.
• On failure:
  • mysqli_query() returns false. You can use mysqli_error() to get a description of the error.

Example

1. Executing a SELECT Query

<?php
// Step 1: Connect to the database
$conn = mysqli_connect('localhost', 'username', 'password', 'database');

if (!$conn) {
    die("Connection failed: " . mysqli_connect_error());
}

// Step 2: Prepare and execute a SELECT query
$sql = "SELECT * FROM users";
$result = mysqli_query($conn, $sql);

if ($result) {
    // Fetch and display rows
    while ($row = mysqli_fetch_assoc($result)) {
        echo "Username: " . $row['username'] . " - Email: " . $row['email'] . "<br>";
    }
    
    // Free the result set
    mysqli_free_result($result);
} else {
    echo "Error: " . mysqli_error($conn);
}

// Step 3: Close the connection
mysqli_close($conn);
?>

2. Executing an INSERT, UPDATE, or DELETE Query

<?php
// Step 1: Connect to the database
$conn = mysqli_connect('localhost', 'username', 'password', 'database');

if (!$conn) {
    die("Connection failed: " . mysqli_connect_error());
}

// Step 2: Prepare and execute an INSERT query
$sql = "INSERT INTO users (username, email) VALUES ('john_doe', 'john@example.com')";
if (mysqli_query($conn, $sql)) {
    echo "New record created successfully";
} else {
    echo "Error: " . mysqli_error($conn);
}

// Step 3: Close the connection
mysqli_close($conn);
?>

Notes

• Error Handling: Always check if mysqli_query() returns false to handle potential errors. Use mysqli_error() to get the error message.
• Security: To avoid SQL injection, especially when using user inputs in queries, use mysqli_real_escape_string() to escape special characters, or use prepared statements.