Some PHP I wrote for an invoice generator. /r/badcode

gistfile1.txt

function mysqliInsert($table, $fields, $optional){
	$mysqli = new mysqli("localhost", "dbUser", "dbPass", "dbName"); //connect to database
	$arrayLen = count($fields);
	$query = "INSERT INTO $table("; //Generate Insert Statement
	$i = 1;			
	foreach ($fields as $field => $value) { //Fields here
		$query .= $field;
		if($i < $arrayLen){ //To remove trailing comma
			$query .= ", ";
		};
		$i++;
};

	$query .= ") VALUES ("; // Values go here
	$i = 1;			
	foreach ($fields as $field => $value) {
		$query .= "'" . $value . "'";
		if($i < $arrayLen){ 
			$query .= ", ";
		}
		$i++;
	}
	$query .= ");";
	if($result = mysqli_query($mysqli, $query)){
		if(array_key_exists('extra', $optional)){
			return mysqli_insert_id($mysqli);
		};	
	};
};


$i = 0;
foreach($_POST['service'] as $key => $value){  
	if($value !== ''){
		$table = 'billedServices';
		$fields = [
			'description' => $value,
			'cost' => $_POST['cost'][$i],
			'invoiceNo' => $lastId
		];
		$optional = [];
		mysqliInsert($table, $fields, $optional);
	};
	$i++;
};