How to replace quotes-herringbones with character code in php?

Question

XML

The requirements for exporting news to rss for the Yandex News service say the following:

Characters < > & ' " occurring in the text must be replaced with the corresponding
elements:
...
Quotation marks-"Christmas trees" - character codes 171 and 187

My texts are printed by a typographer, so the quotation marks are Christmas trees. To replace quotes, I used the htmlentities function, but as a result, the xml output turned out to be incorrect:
Entity 'laquo' not defined
I.e. Christmas trees were replaced with laquo, and not with a character code (171 or 187). How can this be fixed?