One of my recent clients hosting has him on php4.3 and they told him in order to update him to php 4 or above they have to migrate him to a whole new server.
My opinion is that any decent hosting company would keep their servers up to date or at least have the option to choose which version you want to run.
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If you have the issue still there are a couple other fixes as well. One is to edit the socialgroups files I believe and the other is to edit the class_core file. I will find these fixes if you need them. I believe they are at Vbulletin.com.
I believe the minimum on 4.2.3 is technically PHP 5.4~, as Facebook's new API requires it.