<?php

$a=null;

function dbName(){
	global $a;

	$a = 'subsql';
	var_dump($a);

	$a = 'subsql1';
	var_dump($a);
}

function isDB(){
	global $a;
	dbName();
	var_dump($a);
}

isDB();

string(6) "subsql"
string(7) "subsql1"
string(7) "subsql1"