Software Developer

XOR approach can be used to solve this question in an efficient manner. The following property of XOR can be used :
1. If x1^x2^…xn = a and x1^x2^..xn-1 = b , then a^b = xn

Steps:
1. Declare two variables a= 0 and b = 0
2. Calculate the xor of numbers from 1 to n and store them in a i.e. 1^2^…n = a. 
3. Now traverse the array from start to end.
4. For every index i update b as b = b ^ arr[i]
5. Return the missing number as a ^ b. 
Time Complexity : O(N)
Space complexity : O(1)

This problem can be solved using the hashing concept. The objective is to run over the string and record how many times each character appears in it in a hash map. Then we run over the string again, this time using the hash map as a reference to see if any of the characters are unique. If the character is unique, return the index.
Time Complexity: O(N) 
The above approach requires two traversals of the string. To solve the question in a single traversal only, along with storing the count of a character store the index a particular character was encountered the first time. So when it comes to finding the first non-repeating character, just scan the hash map, instead of the string and return the character with occurrences as 1 and least index in the string.

A connection pool is a type of database connection cache that can be customized to meet unique needs. Connection pooling is a well-known data access method that aims to reduce the overhead of database connections and read/write database operations.

class DataBaseConnector {

private static $obj;

private final function __construct() {
echo __CLASS__ . " initialize only once ";
}

public static function getConnect() {
if (!isset(self::$obj)) {
self::$obj = new DataBaseConnector();
}

return self::$obj;
}
}

$obj1 = DataBaseConnector::getConnect();
$obj2 = DataBaseConnector::getConnect();

var_dump($obj1 == $obj2);
?>

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

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

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

The four types of PHP errors are:
1. Warning Error : A warning error in PHP does not stop the script from running. It only warns you that there is a problem, one that is likely to cause bigger issues in the future.
2. Notice Error : Notice errors are minor errors. They are similar to warning errors, as they also don’t stop code execution.
3. Parse Error : Parse errors are caused by misused or missing symbols in a syntax. The compiler catches the error and terminates the script.
4. Fatal Error : Fatal errors are ones that crash your program and are classified as critical errors. An undefined function or class in the script is the main reason for this type of error.

When a user request a web page e.g. p1.php, session_start() function is used to create new session. The session ID is sent to browser, where its stored as cookie. When user send another request to same web application, session ID cookie is automatically added to HTTP request packet, which is used by server to identify particular session of the user from many others.