I remember candace telling stacey on the phone that her brother phineas and her stepbrother ferb never get in trouble(ep.1 rollercoaster). that means that phineas and ferb have a diff. father and it's not lawrence.
Their father is unknown, yes, Linda married Lawrence and so while Phineas and Candace are her biological children, Ferb is not. Nothing is known about their dad though. Something may have happened to him. The creators themselves probably haven't thought of it, they created the mixed family. It does Показать that Phineas and Candace were pretty young when Ferb came to live with them.