It appears that there is a US Customs preclearance facility at YHZ. If you clear customs/immigration there, then you are considered a domestic passenger and you can check your bag through to LAX. You will not have to reclaim it and re-check it at PHL.

90 minutes should be plenty of time for this connection, provided your YHZ-PHL flight is reasonably on time. As soon as you arrive at PHL, look for the nearest set of TV screens showing departing flights. Look for your PHL-LAX flight and go to that terminal/gate.

You don't say which airline you are flying, but I presume it is US. If that is the case, then you may arrive at Terminal F (this is where the regional jets usually end up). Your PHL-LAX flight will leave from Terminal B/C. Take the free inter-terminal shuttle bus; this will allow you to get from F to B/C without having to go through security again in PHL.