`php_htmlInStrings` causes incorrect highlighting for `<?php $x = '<?php'; //remaining statements`

[TysonAndre]

php.vim inherits from the syntax for html.vim, so when html is put inside of a php string, that allows starting a new php region string for syntax.

Having a new option to just support a basic subset of html for php_htmlInStrings may be more appropriate.

syn cluster htmlPreproc add=phpRegion,phpRegionAsp,phpRegionSc

The htmlPreproc is needed for <a href="<?= htmlentities($url) ?>?> to be highlighted.