Need to regulate voltage from 5.8 to 5 without battery. Any ideas?

The absolute simplest circuit would be just to put a slightly less than 5 volt zener diode in parallel with the panel output. If the zener is sized to handle ~10 watts, it would draw enough current from the panel to keep the voltage from rising above the knee voltage of the diode. Any attempt by the real load to draw current at that voltage would tend to reduce the voltage from the panel to the point where the zener would draw less and less current.
As the load tried to pull more current, the voltage would drop enough that the zener was not conducting at all and all of the panel power was going to the load.

Since you do not have anything else to do with the excess power from the panel, the wasted energy would be of no concern.

My guess is that with a properly sized zener, you could go from full current to zener to full current to load over a range of only one or two tenths of a volt.

First, thank you for responding. This sounds promising from what I've just researched about it, but I cannot seem to find a zener diode that fits the specs. I've tried to source it as a 4.75-5.1 volt (ex.) 10 watt zener diode and every other way I could figure with no luck. I'm lost, could you point me in the right direction. Thanks again.

It is not going to work as the voltage is too low to be of any use. For a solar USB charger you are going to need at least 20 watts @ 7 to 10 volts panel. However will be limited to only 1 or 2 hours daily charge time unless you have a tracker. A series or shunt voltage regulator requires the input voltage to be at 1.5 to 2 volts higher than output.

I works just fine for everything but my iPhone right now, and all you need for that is 500mA and a steady 5V. I am pulling >500mA at 5V even on fairly cloudy days all day long less 1-2 hours split between sun rise/set. I do this with my 10W semi flexible panel laid flat with no tracker. On a sunny day I have enough to charge 3 phones just fine. Right now it will charge my iPhone if I plug in a Micro-USB phone (Samsung, HTC, etc.) first, which drops/steadies the voltage from 5.8 to 4.8 and works great. I have solved the issue long term with not using 5V panels anymore and using a converter. I would just like to know if anyone knows how to regulate my 5.8V output or even 'waste' a volt on the way to the hub so I can use existing panels without issues.

That is what I am trying to tell you. It needs 2 amps, the new USB charging standard is 2 amps, and you have to short pins 2 and 3 together. Read up on USB charging standard. Easiest way to make a USB solar charge is to buy a 12 volt vehicle 2 amp USB charger for $7 and connect it directly to a 40 or higher watt 12 volt battery panel. (A panel with a Isc of 2 amps or greater)

My 10W 18V panel ran thru my 12-24V to 5V dc/dc converter to my USB hub I have had sourced to be power only works AMAZING RIGHT NOW WITH ALL PHONES. You just pulled an old media trick and quoted half of a sentence, out of context. I don't want you to bury my only issue with you being hung up on technicality issues. I appreciate your concerns, but the thread was not 'Will this work?' I know it works. It's about trying to regulate my voltage from 5.8 to 5.0 and if it is even possible.

If you do not see exactly what you want, you can put it together by putting more than one Zener in series, or adding one or more forward biased conventional rectifier diodes or Schottky diodes in series with a zener.

Or you could just go the other route and put a forward biased rectifier in series between the panel and the load to drop ~.7 volts and a Schottky diode to drop another .2 volts. This would allow you to take the higher voltage for battery chargers that will accept it and still deliver the lower voltage on the cable to the iphone. Easy to try to see whether it works. Lower power requirements on the diodes, since they will be sinking no more than 1.5 watts at 2 amps.

You can have several diodes in series to drop several Volts.

Use an L7805 to limit the output Voltage, which you can feed up to 37.5V

You might find the 7805 doesn't have enough input Voltage to begin with at 5.8V, (I seem to remember they drop 1.something Volts) you'll have to experiment...

You are too easy. Very simple, use any Plain Jane Silicon diode in series. All SI diodes have a .7 volt forward bias drop. Select any that can handle your current and at .5 amps is about any silicone diode. USB is not picky. It will work anywhere from 6 to 5 volts on any universal USB charging port. Apple is the only exception as it must have 2 amps with pins 2 and 3 shorted together. [...] The new USB standard today is 2 amps for all new devices. Made over a dozen of them.

5.8 volts in > Diode > 5.1 volts out

Apple has done some even more egregious things in regard to their new "Lightning" connector standard (smaller footprint) for their new iDevices.
They will only license the connector for use by third party charger and adapter manufacturer if the Lightning connector is the only output the charger has. They will not even allow a charger which works for both the old and the new iDevices. And of course, even Apple themselves do not make a dual output charger for the old and new connectors. They do make and license adapters from 30 pin to Lightning, but even these have a proprietary ID chip in them which handshakes with the device to tell it that it is working with a licensed accessory. And there are no Lightning back to old standard adapters that I have found.

The new USB power standard allows power-only USB chargers/supplies to deliver even higher voltages and/or higher currents, but only after handshaking with the load to negotiate what sort of power the charger is expected to deliver. (Shorting 2-3 is an early rudimentary handshake, I guess.) That handshake allows the exact same connector to be used for both the old and new USB power connections without interoperability problems.

Here you go. A USB to fast charge anything [...] using TI TPS54240. If you try to make one would cost you around $50 after you sourced all the parts and have a board etched. Or spend $9 to $20 for a DC 12 volt version that will accept 8 to 24 volts. At $9 would be foolish to even try to fabricate a home grew. It will detect from the device the appropiate current of 500 ma, 1 amp, or 2.1 amps. Piece of cake. Want solar, just connect at least a 20 watt 12 volt battery panel, but I recommend at least 40 watts.