What are the differences between htmlentities, htmlspecialchars, and addslashes when working with MySQL in PHP?

When working with MySQL in PHP, it is important to properly sanitize user input to prevent SQL injection attacks. htmlentities, htmlspecialchars, and addslashes are functions that help sanitize input, but they serve different purposes. htmlentities and htmlspecialchars are used to encode special characters in a string to prevent XSS attacks, while addslashes is used to escape characters that have special meaning in SQL queries. It is recommended to use htmlspecialchars or htmlentities when displaying user input in HTML, and addslashes when inserting user input into a SQL query.

// Using htmlspecialchars to sanitize input for display in HTML
$user_input = "<script>alert('XSS attack');</script>";
$sanitized_input = htmlspecialchars($user_input, ENT_QUOTES);

echo $sanitized_input;
```

```php
// Using addslashes to sanitize input before inserting into a SQL query
$user_input = "John's Pizza";
$sanitized_input = addslashes($user_input);

$query = "INSERT INTO table_name (column_name) VALUES ('$sanitized_input')";
$result = mysqli_query($connection, $query);