Don't muddy up the $_GLOBAL space. If you want to nix the class route and use a function with a statically scoped variable you can keep it contained.

// getter + setter function 

function prefix_my_var( $value = null ) {
    static $s = null;
    if ( ! is_null( $value ) ) {
        $s = $value;
    }

    return $s;
}

// set + get

$value = prefix_my_var( 'foo' ); // foo

//  set

$value = prefix_my_var();        // foo

Now if you pass a value it'll set+get it. And if you don't pass a value it'll just get the current value.

echo "<pre>";
print_r( array (
        prefix_my_var(),
        prefix_my_var( 'foo' ),
        prefix_my_var(),
        prefix_my_var( 'bar' ),
        prefix_my_var(),
        prefix_my_var( 'baz' ),
    ) );

Array
(
    [0] => 
    [1] => foo
    [2] => foo
    [3] => bar
    [4] => bar
    [5] => baz
)

Don't muddy up the $_GLOBAL space. If you want to nix the class route and use a function with a statically scoped variable you can keep it contained.

// getter + setter function 

function prefix_my_var( $value = null ) {
    static $s = null;
    if ( ! is_null( $value ) ) {
        $s = $value;
    }

    return $s;
}

// set + get

$value = prefix_my_var( 'foo' ); // foo

//  get

$value = prefix_my_var();        // foo

Now if you pass a value it'll set+get it. And if you don't pass a value it'll just get the current value.

echo "<pre>";
print_r( array (
        prefix_my_var(),
        prefix_my_var( 'foo' ),
        prefix_my_var(),
        prefix_my_var( 'bar' ),
        prefix_my_var(),
        prefix_my_var( 'baz' ),
    ) );

Array
(
    [0] => 
    [1] => foo
    [2] => foo
    [3] => bar
    [4] => bar
    [5] => baz
)