How can I get the definition (tooltip) of a core PHP function to show up in phpStorm? Follow

Typically when I type the first parentheses the arguement list for that function comes up and tells me what goes where. I say typically because sometimes for what seems like no reason at all the definition won't appear, so I end up either looking the function up on the web, or pressing ctrl-b when the cursor is over the function which loads the PHP header files. I'd rather not do that and just be able to see the tooltip when I need to, but I don't know which hotkey that is. Does anyone know what I have to press to show the definition? I'm just talking about functions like explode or str_replace because I constantly forget the order of the arguments for these functions.