Understanding Global Variables in PHP

Global variables in PHP are declared using the global keyword, followed by the variable name. The exact syntax is as follows: global $variable;. This means that the correct answer to the quiz question is global $variable;.

To understand it clearly, let's delve into the concept of global variables in the PHP programming language and see an example.

Global variables in PHP are variables that can be accessed from any part of the script, not just from the part of the script where they were declared. For a PHP variable to be accessible globally, it needs to be specifically declared as a global variable using the global keyword.

Imagine you have a function, and within this function, you need to access a variable that was declared outside of it. Without declaring the variable as a global one, the function would not be able to access it, because by default, all variables declared outside of a function are local to the script in which they're declared, and all variables declared inside a function are local to that function.

Here is an example:

$x = 5;
$y = 10;

function multiply() {
  global $x, $y;
  $y = $x * $y;
}

multiply();
echo $y; // outputs: 50

In the code above, $x and $y were declared globally. Inside the multiply function, we then declared them as global which allows us to access them within the function. After calling the function, the value of $y has been changed to 50.

As with any powerful feature of a programming language, global variables must be used judiciously. Though they are available everywhere in the code, overusing them can lead to code that's hard to debug and maintain. If the codebase is large and the global variable is used or modified in many different places, it can be difficult to track down and understand all of its uses.

Remember that the best practice is to always keep the scope of your variables as small as required. Resort to global variables only if necessary!