How do you write a comment in PHP?

The `strlen()` function in PHP returns the length of a string, measured in characters. For example: `strlen('Hello world');` will return `11`. It counts the number of characters in the string, including spaces and special characters. This function is useful for determining the size of a string, validating input lengths, or managing text-based data.

To remove whitespace from a string in PHP, use the `trim()` function which removes whitespace from the beginning and end of a string. For example: `trim('  Hello world  ');` will return `'Hello world'`. Additionally, `ltrim()` can be used to remove whitespace from the beginning, and `rtrim()` from the end of a string. These functions help clean up user input and format text.

To validate email addresses in PHP, use the `filter_var()` function with the `FILTER_VALIDATE_EMAIL` filter. For example: `if (filter_var($email, FILTER_VALIDATE_EMAIL)) { echo 'Valid email'; } else { echo 'Invalid email'; }`. This function checks the syntax of the email address and ensures it conforms to standard email formats. It's an effective way to validate user inputs.

The `array_push()` function in PHP adds one or more elements to the end of an array. For example: `array_push($array, 'new_value');` will append 'new_value' to the end of `$array`. It can also accept multiple values: `array_push($array, 'value1', 'value2');`. This function is useful for dynamically adding items to arrays.

The `json_encode()` function in PHP converts a PHP value (such as an array or object) into a JSON format. For example: `json_encode(array('name' => 'John', 'age' => 30));` will produce `'{'name':'John','age':30}'`. This function is used for encoding data to be sent to a client-side application or stored in JSON format, facilitating data interchange between different systems.

'mysqli_prepare()' is used in PHP to prepare an SQL statement for execution, allowing for parameterized queries that enhance security. For example: `$stmt = mysqli_prepare($conn, 'SELECT * FROM users WHERE email = ?');`. Placeholders (like `?`) are used in the query, and then parameters are bound using `mysqli_stmt_bind_param()`. This approach helps prevent SQL injection by separating the SQL code from the data.

The `strip_tags()` function in PHP removes HTML and PHP tags from a string. For example: `strip_tags('<p>Hello</p>');` will return `'Hello'`. This function is useful for sanitizing user input by removing unwanted tags and preventing potential security risks, such as cross-site scripting (XSS) attacks. It is often used when displaying user-generated content.

The `file_get_contents()` function in PHP reads the entire content of a file into a string. For example: `$content = file_get_contents('file.txt');` reads the contents of 'file.txt' and stores it in the variable `$content`. This function is commonly used for reading files, fetching data from URLs, or working with file content in a simple way.

The `uniqid()` function in PHP generates a unique identifier based on the current time in microseconds. For example: `uniqid();` might produce a string like `'5f0e0d8b5e4b1'`. This function is often used to create unique keys or identifiers for objects or sessions. It can also accept a prefix string to prepend to the generated ID.

The `array_merge()` function in PHP combines multiple arrays into one. For example: `array_merge(array('a', 'b'), array('c', 'd'));` will result in `array('a', 'b', 'c', 'd')`. This function merges the arrays in the order they are passed, with the values of subsequent arrays appending to the first array. It is useful for aggregating data from multiple sources.

'mysqli_fetch_assoc()' fetches a result row as an associative array from a MySQL database query. For example: `while ($row = mysqli_fetch_assoc($result)) { echo $row['column_name']; }` retrieves rows from a result set where each row is an associative array with column names as keys. This function is useful for accessing query results in a readable format.

The `empty()` function in PHP checks if a variable is empty. It returns `true` if the variable is empty (i.e., '', 0, '0', NULL, FALSE, array()), and `false` otherwise. For example: `if (empty($var)) { echo 'Variable is empty'; }`. This function is useful for validating variables and managing conditional logic based on variable content.