"a phototransistor would be more suitable, because it is much more sensitive than a photodiode,"
Sorry, a photodiode better way to go. The phototransistor is essentially a slow photodiode with much more capacitance.
But heck, either will work for a simple light interrupter where response time not an issue. But a PD with an op-amp
in transimpedance circuit is the standard way to go.
And he doesn't need a laser, any old led will work.