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When I read this type of post, generally my reaction is: if you want to follow the crowd; go ahead, no one is stopping you.
But your particular formulation of this question is somewhat different to most. You appear to be saying that your customers are telling you that you should be developing their web sites using PHP.
That leaves me with one question. If they want PHP, and you use Perl, why are they your customers?
The other thing that strikes me from your post is:
The immense popularity of PHP due to open-source systems like WordPress, Joomla, Typo3, and Drupal have raised up a new crop of PHP coders, lots of them (see the chart on my personal node). From the looks of it, you could throw a rock in any direction and hit a PHP coder.
My reaction to that--as a staunchly non-web perler--is: do you want to be a small fish in a big pond; or a big fish in a small pond?
If you believe, given your knowledge of your customer base, that you can either, earn more money; or derive more satisfaction; by coding your customers web sites using PHP, then do that. If not, don't.
There is no point in being an Edsel salesman in a Mercury marketplace.
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.