In PHP, variable scope refers to the visibility and accessibility of variables within a program. Variables can be either global or local, depending on where they are declared and how they are used.

Global variables are variables that are defined outside of any function or class, and are available for use throughout the entire program. They can be accessed and modified from any function or class in the program, regardless of where they are defined.

Here is an example of a global variable in PHP:

$x = 10;

function foo() {
global $x;
echo $x; // Outputs 10
}

foo();

In this example, the $x variable is defined as a global variable outside of any function or class. It is then accessed and modified within the foo() function using the global keyword. The foo() function outputs the value of the $x variable, which is 10.

Global variables are generally considered bad practice in programming, as they can lead to problems with code organization and maintenance. They can also cause conflicts with variables of the same name that are defined in other parts of the program.

Local variables, on the other hand, are variables that are defined within a function or class, and are only available for use within that function or class. They cannot be accessed or modified outside of the function or class in which they are defined.

Here is an example of a local variable in PHP:

function foo() {
$x = 10;
echo $x; // Outputs 10
}

foo();
echo $x; // Undefined variable

In this example, the $x variable is defined as a local variable within the foo() function. It is only available for use within the foo() function, and cannot be accessed or modified outside of the function. If you try to access the $x variable outside of the foo() function, you will get an error message saying that the variable is undefined.

Local variables are the recommended way to define variables in PHP, as they improve code organization and reduce the risk of conflicts with other variables.

You can also use the static keyword to create a static variable within a function. A static variable is a local variable that retains its value between function calls. This means that the value of a static variable is preserved between function calls, and is not reset to its default value each time the function is called.

function foo() {
static $x = 0;
$x++;
echo $x;
}

foo(); // Outputs 1