Function Description 
define("GREETING","Hello you! How are you today?"); 
echo constant("GREETING");
 The define() function defines a constant.
Constants are much like variables, except for the following differences:
1) A constant's value cannot be changed after it is set
2) Constant names do not need a leading dollar sign ($)
3) Constants can be accessed regardless of scope
4) Constant values can only be strings and numbers 
mysqli_connect(host, username, password, dbname, port, socket);
Parameter Description
host Optional. Specifies a host name or an IP address
username Optional. Specifies the MySQL username
password Optional. Specifies the MySQL password
dbname Optional. Specifies the default database to be used
port Optional. Specifies the port number to attempt to connect to the MySQL server
socket Optional. Specifies the socket or named pipe to be used
Return Value: Returns an object representing the connection to the MySQL server
If return value is false than connection is not successfull 
require_once 'config.php';
 The require_once keyword is used to embed PHP code from another file. If the file is not found, a fatal error is thrown and the program stops. If the file was already included previously, this statement will not include it again. 
if (empty($a)) {
    echo "Variable 'a' is empty.
";
}
 The empty() function checks whether a variable is empty or not.
This function returns false if the variable exists and is not empty, otherwise it returns true. 
trim($str);
 The trim() function removes whitespace and other predefined characters from both sides of a string. 
if ($_SERVER["REQUEST_METHOD"] == "POST") {
    // collect value of input field
    $name = $_POST['fname'];
    if (empty($name)) {
        echo "Name is empty";
    } else {
        echo $name;
    }
}
 PHP $_POST is a PHP super global variable which is used to collect form data after submitting an HTML form with method="post". $_POST is also widely used to pass variables. 
mysqli_prepare(connection, query)
Parameter Description
connection Required. Specifies the MySQL connection to use
query Required. Specifies an SQL query. Note: Do not add semicolon to the end of the query!
Return Value: A statement object on success. FALSE on failure 

$servername = "localhost";
$username = "username";
$password = "password";
$dbname = "myDB";

// Create connection
$conn = new mysqli($servername, $username, $password, $dbname);

// Check connection
if ($conn->connect_error) {
  die("Connection failed: " . $conn->connect_error);
}

// prepare and bind
$stmt = $conn->prepare("INSERT INTO MyGuests (firstname, lastname, email) VALUES (?, ?, ?)");
$stmt->bind_param("sss", $firstname, $lastname, $email);

// set parameters and execute
$firstname = "John";
$lastname = "Doe";
$email = "john@example.com";
$stmt->execute();

$firstname = "Mary";
$lastname = "Moe";
$email = "mary@example.com";
$stmt->execute();

$firstname = "Julie";
$lastname = "Dooley";
$email = "julie@example.com";
$stmt->execute();

echo "New records created successfully";

$stmt->close();
$conn->close();
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$a = 0;
// True because $a is set
if (isset($a)) {
  echo "Variable 'a' is set.
";
}

$b = null;
// False because $b is NULL
if (isset($b)) {
  echo "Variable 'b' is set.";
}
 The isset() function checks whether a variable is set, which means that it has to be declared and is not NULL. This function returns true if the variable exists and is not NULL, otherwise it returns false. 
$_SERVER['']
Element/Code Description
$_SERVER['PHP_SELF'] Returns the filename of the currently executing script
$_SERVER['GATEWAY_INTERFACE'] Returns the version of the Common Gateway Interface (CGI) the server is using
$_SERVER['SERVER_ADDR'] Returns the IP address of the host server
$_SERVER['SERVER_NAME'] Returns the name of the host server (such as www.w3schools.com)
$_SERVER['SERVER_SOFTWARE'] Returns the server identification string (such as Apache/2.2.24)
$_SERVER['SERVER_PROTOCOL'] Returns the name and revision of the information protocol (such as HTTP/1.1)
$_SERVER['REQUEST_METHOD'] Returns the request method used to access the page (such as POST)
$_SERVER['REQUEST_TIME'] Returns the timestamp of the start of the request (such as 1377687496)
$_SERVER['QUERY_STRING'] Returns the query string if the page is accessed via a query string
$_SERVER['HTTP_ACCEPT'] Returns the Accept header from the current request
$_SERVER['HTTP_ACCEPT_CHARSET'] Returns the Accept_Charset header from the current request (such as utf-8,ISO-8859-1)
$_SERVER['HTTP_HOST'] Returns the Host header from the current request
$_SERVER['HTTP_REFERER'] Returns the complete URL of the current page (not reliable because not all user-agents support it)
$_SERVER['HTTPS'] Is the script queried through a secure HTTP protocol
$_SERVER['REMOTE_ADDR'] Returns the IP address from where the user is viewing the current page
$_SERVER['REMOTE_HOST'] Returns the Host name from where the user is viewing the current page
$_SERVER['REMOTE_PORT'] Returns the port being used on the user's machine to communicate with the web server
$_SERVER['SCRIPT_FILENAME'] Returns the absolute pathname of the currently executing script
$_SERVER['SERVER_ADMIN'] Returns the value given to the SERVER_ADMIN directive in the web server configuration file (if your script runs on a virtual host, it will be the value defined for that virtual host) (such as someone@w3schools.com)
$_SERVER['SERVER_PORT'] Returns the port on the server machine being used by the web server for communication (such as 80)
$_SERVER['SERVER_SIGNATURE'] Returns the server version and virtual host name which are added to server-generated pages
$_SERVER['PATH_TRANSLATED'] Returns the file system based path to the current script
$_SERVER['SCRIPT_NAME'] Returns the path of the current script
$_SERVER['SCRIPT_URI'] Returns the URI of the current page