The variable num is not a global variable. Why can it be called by a function?

$num = 10;

##//Call function A() once;

A($ num);

#function A( $arg ){

echo $arg;

//Go inside function A and run to execute function B

B( $arg);

## echo 'We need to keep working hard until God is moved by us';

echo $arg.'<br />';

}

##function B( $number ){

## echo $number;

## echo 'I am a dog, the execution is finished<br />';

##}