PHP - Converting a UTF-8 string to \x escaped form

My files contain UTF-8. I want to convert UTF-8 characters in the text to \x sequences and preserve everything else.

For example, the text

# Printing 'naïve' and '男孩' with newlines and tabs
    print 'naïve\n'
    print '\t男孩'

should be converted to

# Printing 'na\xc3\xafve' and '\xe7\x94\xb7\xe5\xad\xa9' with newlines and tabs
    print 'na\xc3\xafve\n'
    print '\t\xe7\x94\xb7\xe5\xad\xa9'

I tried the following approach:

mb_convert_encoding($text, "UTF-8", "ASCII");

This resulted in

# Printing 'naïve' and 'ç·å­©' with newlines and tabs
    print 'naïve\n'
    print '\tç·å­©'

(not quite what I was looking for)

I also tried:

iconv("UTF-8", "ASCII", $text);

This resulted in the error:

PHP Notice:  iconv(): Detected an illegal character in input string

How do I get around this?