One of the beta testers for Foliopress WYSIWYG has just complained that Foliopress WYSIWYG is not compatible with PHP4. Apparently PHP5 is still only 6% of the installed PHP base across all webhosts.
That figure should be enough to strike terror into any developer. But that number will change very soon as PHP4 has hit the end of the line.
PHP4 incompatibility started off not as a deliberate decision. Generally I like wider compatibility.
But on serious consideration, I’m not worried about Foliopress WYSIWYG being PHP5 only.