Cannot access global variable from within class constructor - php

Question

Alright so I am trying to simply check if there is something inside the array from construct but it doesnt seem to work at all...
$DB_VALID = array("mysql");
class DB {
function __construct($conn) {
if(in_array($conn,$DB_VALID)) {
echo "exists!";
}
else {
echo "doesnt exist";
}
}
}
Now, since construct is inside a class, if I dump it I will get results saying NULL, but if I dump it outside the construct I will simply get the real results...
Usage
$conn = new DB("mysql");
Results?
in_array returns false

Answer 1

The variable $DB_VALID does not exist inside the __construction function scope.
The recommended solution is that you move $DB_VALID to a static variable inside the DB class, as such:
class DB {
static $DB_VALID = array("mysql");
function __construct($conn) {
if(in_array($conn,self::$DB_VALID)) {
echo "exists!";
}
else {
echo "doesnt exist";
}
}
}
You can later access that array in other parts of your code by referencing it as DB::$DB_VALID.
However, if you must keep the global variable and access it from within __construct, you can use the global keyword to bring it into the local scope, as such:
$DB_VALID = array("mysql");
class DB {
function __construct($conn) {
global $DB_VALID; // Brings the global variable to local scope
if(in_array($conn,$DB_VALID)) {
echo "exists!";
}
else {
echo "doesnt exist";
}
}
}
Please consider the first solution on the future, though, as using global variables is an easy way to have your applications break as they evolve.
Edit: As you mentioned in the comments your restriction is the order you're loading your scripts right now, you should also really look into class autoloading and namespaces, as your projects will get increasingly complex and harder to manage otherwise (see sitepoint.com/autoloading-and-the-psr-0-standard).

Answer 2

If you have to, use the global keyword.
$DB_VALID = array("mysql");
class DB {
function __construct($conn) {
global $DB_VALID; // Makes the variable available in the scope
if(in_array($conn,$DB_VALID)) {
echo "exists!";
}
else {
echo "doesnt exist";
}
}
}
$conn = new DB("mysql"); // will print "exists!"
Note that global variables usually hints organisation problems, so you should probably review your structure and see if it's really necessary to use global here.