It appears that there is a non-breaking space at the start of every PHP code block formed by the [php] forum bbcode.
This does not happen for code blocks created by the [code] forum bbcode.
An inspection of clientside source code seems to place the PHP code block after a comment, "<!-- php buffer start -->". <code> is used, followed by nested <font>s. Immediately after the second <font> lies a nbsp; that should not be there.
The clientside source for the normal code blocks use <pre> and <div> - but I dont think that the use of <code> and <font> is the problem, the problem is just the space.