Encode ’ to be XML safe - php

Question

I have a string that contains a right single quotation mark:
$str = "David’s Spade";
I am sending the string via XML and need to encode it. I have read that I should encode string using htmlspecialchars, but I have found that XML request still fails whereas htmlentities works.
When I error_log $str:
$str; // David\xe2\x80\x99s Spade
htmlspecialchars($str); // David\xe2\x80\x99s Spade
htmlspecialchars($str, ENT_QUOTES, 'UTF-8'); // David\xe2\x80\x99s Spade
htmlentities($str); // David’s Spade
Would it be better to str_replace ’ and then use htmlentities? Are there any other chars htmlentities may miss?

Answer 1

I am sending the string via XML and need to encode it.
No, you don't. If the XML is UTF-8 encoded (it is by default) and as your $str is UTF-8 encoded (as you show by the binary sequences in your question), you do not need to encode it.
This is by the book. So given on the technical information of the data you collaborate with, this is clear and fine.
You then write that some things work and others won't. Whatever you do there, there problem lies within the things you've hidden from your question.
To make this more explicit:
$str = "David’s Spade"; // "David\xE2\x80\x99s Spade"
is a perfectly valid string, for example to use it with an XML library like Simplexml to add it to an XML document:
$xml = new SimpleXMLElement('<?xml version="1.0" encoding="UTF-8"?><doc/>');
$xml->element = $str;
$xml->asXML('php://output');
Output:
<?xml version="1.0" encoding="UTF-8"?>
<doc><element>David’s Spade</element></doc>
As you can see, the XML has been encoded by not changing the byte-sequence of the string here because it's UTF-8.
Let's take some ASCII:
$xml = new SimpleXMLElement('<doc/>');
$xml->element = $str;
$xml->asXML('php://output');
Output:
<?xml version="1.0"?>
<doc><element>David’s Spade</element></doc>
As this example shows, it depends on the document encoding then. This second example is a fall-back of Simplexml to make the output more robust, but actually this wouldn't be necessary as UTF-8 would be the default encoding.
In any case you should not be too concerned about the encoding yourself by using a library that has specialized on creating XML documents. PHP has some few for exactly that. Take one of them.