Depends on the solar panel. It needs to have a USB port that provides enough electricity to charge the iPhone (which is a function of both the panel’s / port’s maximum output and how much uninterrupted sunlight it gets).
In my own experience with a small 14 Watt solar panel it’s better to have the panel charge a battery pack (partially pre-charged), while that pack charges the iPhone. Directly charging from the panel causes the iPhone to frequently stop and restart charging as the Wattage fluctuates.
A higher-power solar panel would fair better in a direct charging scenario. I wouldn’t even try a lower-powered one.
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