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I'm currently following this guide on making an Arduino OpenTherm Controller. After finding almost all of the parts without any issue; my only question remained with the "PNP transistor" named in the hardware list. As someone who is new to electronics, this did not mean much to me, so I sought out an exact name for this transistor. Elsewhere on the webpage I found this image:

As you can see, it names the transistor as a BC558A. However, after searching online for this transistor I either found few or no results. On the other hand, I found many BC558B transistors like this one so I ordered a pack. After initially thinking that because no specific "PNP transistor" was named; any sort would do, I began to reconsider this which is why I am asking for help.

My question is: Will the BC558B transistor I have ordered function in the same way in the circuit? If not, what would a reasonable alternative to the BC558A transistor be, seeing as it is very hard to find online?

Thank you in advance for any help.

tl;dr For this circuit, it doesn't appear to matter at all. For a precision analog circuit, it will likely matter.

For the BC558 transistor in particular, the datasheet indicates the following behavior for the Hfe parameter:

This parameter relates the collector current to the base current as a ratio. Parts labeled BC558A are guaranteed to have Hfe within one particular range; parts labeled BC558B are guaranteed to have Hfe in a different range. Beyond that, there are no significant differences--they don't have different shelf life, and all of the other datasheet parameters will still hold across these series.

However, based on the circuit shown in your question, this parameter is not strongly relevant; it appears that the transistor is being driven fully-on or fully-off and as a result this parameter has only a weak effect on the circuit's behavior. Moreover, the transistor is attached to optoisolators that themselves have very imprecise characteristics unless binned in the same way.

However, if you are trying to construct an analog circuit where this parameter is important, you may need to select a particular series of this transistor (A, B, or C) and remain within that series. Even then, many analog topologies are designed to be insensitive to certain transistor parameters to avoid this issue.

if it just says PNP transistor without naming any particular one, it's likely just about any reasonably-selected one will do. You could use a BC558, a 2N3906, you might even get away with a TIP42 if it's not too picky about low β.

Like @isdi said in the comments, a suffix on the end of a part number usually just indicates a better product than the original, but one that is otherwise interchangeable with the original. Think of it like a "version 1.1" of the device.