PHP parsing xml file error

I believe preprocessing XML with regular expressions might be just as bad as parsing it.

But here is a preg replace that removes all non-whitespace characters, from the beginning of the string, from the end of the string, and after closing/self-closing tags:

$string = preg_replace( '~
    (?|           # start of alternation where capturing group count starts from
                  # 1 for each alternative
      ^[^<]*      # match non-< characters at the beginning of the string
    |             # OR
      [^>]*$      # match non-> characters at the end of the string
    |             # OR
      (           # start of capturing group $1: closing tag
        </[^>]++> # match a closing tag; note the possessive quantifier (++); it
                  # suppresses backtracking, which is a convenient optimization,
                  # the following bit is mutually exclusive anyway (this will be
                  # used throughout the regex)
        \s++      # and the following whitespace
      )           # end of $1
      [^<\s]*+    # match non-<, non-whitespace characters (the "bad" ones)
      (?:         # start subgroup to repeat for more whitespace/non-whitespace
                  # sequences
        \s++      # match whitespace
        [^<\s]++  # match at least one "bad" character
      )*          # repeat
                  # note that this will kind of pattern keeps all whitespace
                  # before the first and the last "bad" character
    |             # OR
      (           # start of capturing group $1: self-closing tag
        <[^>/]+/> # match a self-closing tag
        \s++      # and the following whitespace
      )
      [^<]*+(?:\s++[^<\s]++)*
                  # same as before
    )             # end of alternation
    ~x',
    '$1',
    $input);

And then we simply write back the closing or self-closing tag if there was one.

One of the reasons this approach is not safe is that closing or self-closing tags might occur inside comments or attribute strings. But I can hardly suggest you use an XML parser instead, since your XML parser can't parse the XML either.