Don't know about PHP but for MySQL... they are mostly right. I've seen huge systems based on MySQL which are handling billions of $$ every year - and they do work. IMO it's mostly matter of programming culture and less matter of pure SQL-capability of the system used. While MYSQL is not perfect (and neither is PostgreSQL) it does scale, you can make it work - may be not as easily as PostgreSQL, but it can be done: I've seen it done.
PHP... I'm yet to see big, complex, reliable system with heavy use of PHP. Sometimes PHP is used "on the side", but when system becomes complex and PHP is in the middle of it... it's inevitable disaster...
P.S. I've seen huge systems written in Python, Java (of course) and even Ruby (hmm, it works) or Perl (horrors), but PHP... nope. Don't really know why... But may be it's the attitude ? PHP is the only language where "security update" can break half of the scripts on your site and when the question will be raised in the mailing list the answer will be "oh, these scripts where never written to the specifications, so it's Ok to break them - you just need to fix them".
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