PHP is not a very strict language when it comes down to data types.
Strings are values that contain characters. This can include basically everything you can type on your keyboard.
Integers are values that just contain numbers. Notice in the example above that there are no quotes?
Well, you can write an integer like that, or like this:
PHP automatically converts numbers contained in a string with no other characters into a integer. The only time you need to be careful of this is when you are using operators (I will discuss that in another post).
$string2 = false;
Booleans are values that contain either true or false. They can be used in if statements like:
if ($string)//checks if $string is true
echo '$string is true';
echo '$string is false';