No. It was not an attack, it was a conjuration of a wall, that then indirectly damage a target.
" The spell ends if the subject attacks any creature. For purposes of this spell, an attack includes any spell targeting a foe or whose area or effect includes a foe. Exactly who is a foe depends on the invisible character's perceptions. Actions directed at unattended objects do not break the spell. Causing harm indirectly is not an attack. Thus, an invisible being can open doors, talk, eat, climb stairs, summon monsters and have them attack, cut the ropes holding a rope bridge while enemies are on the bridge, remotely trigger traps, open a portcullis to release attack dogs, and so forth."
Wall of Fire is a bit iffy since it has a damaging AoE besides someone simply walking through it, however, so if you conjure it close enough that would automatically occur (or likewise conjure it right ON somebody) it would probably be an attack.
I'm with Rynjin, if I were adjudicating the Wall of Fire if it was conjured on top of someone or if it damaged any opponent of said caster he would immediately become visible. If he saw some folks coming down a hall however and cast the wall of fire I would do a couple of things depending on the distance and the enemy I would roll them perception to see if they can hear the spell being cast or hear the sound of the fire. I picture a wall of fire would make a noise not unlike a roaring fire, lol. Blade barrier would be more ideal than a wall of fire.
Jwes ... they'd just see the wall in either case. The caster being invisible doesn't make the spell effect invisible.