1.
$sum = 0;
for($i = 0; $i < count($arr); $++){
	$sum += $arr[$i];
}

2.
$hostname = 'hostname';
$username = 'username';
$password = 'password';
$dbName = 'dbName';
$table = 'table';
$delId = 'delId';
$mysqli = new mysqli($hostname, $username, $password, $dbName);
$mysqli->query('DELETE FROM '.$table.' WHERE id = "'.$delId.'"');
$mysqli->close();

3.
$path = getcwd();
$objects = scandir($path);
foreach($objects as $file){
	$new_file = $path.'/'.$file;
	if(is_file($new_file)){
		echo $file.'<br>';
	}
}

4.
$array = array(3, 1, 2, 4);
sort($array);
foreach($array as $elem){
	echo $elem.'<br>';
}