DANG DANG DANG !
Well my own fault of course, but as you may know, enclosing everything in double quotes takes just a fraction more time for PHP to parse than text enclosed in single quotes, so I've been trying to get into the habit of using single quotes whenever possible.
As I understand it, when PHP parses a double quoted string, it basically has to run through it twice, once to get the "static" values and once to look for any variables that need converting.
So this ...
$html .= 'Some static text with a '. $variable .' in it';
is preferable to this ...
$html .= "Some static text with a $variable in it";
of course on a single line like this the difference in parsing time is infinitesimally small, but on larger programs it does add up.
Now if I can just remember to get the single quotes on both ends of the line.
