<%= is a unique escape code

It always bugged me that the PHP community didn't pick up on this and run with it. We've had ASP tags since the beginning (almost?) but I think people avoid it out of some anti-MS stance. Rails runs with <%= just fine.

Can you give a demonstration of its non-uniqueness?

<? is used in XML headers.

OTOH, you shouldn't ever be generating XML headers from PHP anyway (XHTML is a dead-end, and raw XML should be generated by an XML library), so this shouldn't be that big of an issue.

But, less than question mark and a space are not a valid XML combination. XML processing instructions cannot begin with a space. Likewise, PHP cannot run commands into the short tags, like <?echo $foo; ?>. So, there really is no collision, it's just grandstanding.

I wonder if the XML PI spec says anything about <?= Can you have a PI start with = ? If not, the echo short tag should stay for certain.

'<?=' is invalid XML. '<?' must be followed by a valid XML Name.

See http://www.w3.org/TR/2008/REC-xml-20081126/#sec-pi