mechanics for 'illusionary spell'

my character took the spell 'illusionary spell' on level-up today. after i listed out all of the neat-o spells it could mimic, i thought, 'can't go wrong there'! so my question is, how does it work, mechanically? if the target fails a will save, will it then become subject to the effects of the real spell? if so, will it need to save again v. whatever spell school the spell is, or is this like 'shades', where the school is illusory and affects the saving throw? for example, suppose i use 'illusionary spell' to cast a fireball. how exactly is damage calculated? and how would the target save v. crispy critter if it actually believes the illusion?

Illusionary Spell is just a roleplay spell, it doesn't actually do anything other than "looks cool". It creates a visualeffect of the spell you selected before casting it.

oh noes!!!
*sigh* well, i guess i did it again. :-/