Try to convert HTML, unicode, and other smart quotes to plain quotes. Fork this gist to add support for more types.

smartquotes.php

<?php

function convert_smart_quotes($string)
{
	$quotes = array(
		"\xC2\xAB"	 => '"', // « (U+00AB) in UTF-8
		"\xC2\xBB"	 => '"', // » (U+00BB) in UTF-8
		"\xE2\x80\x98" => "'", // ‘ (U+2018) in UTF-8
		"\xE2\x80\x99" => "'", // ’ (U+2019) in UTF-8
		"\xE2\x80\x9A" => "'", // ‚ (U+201A) in UTF-8
		"\xE2\x80\x9B" => "'", // ‛ (U+201B) in UTF-8
		"\xE2\x80\x9C" => '"', // “ (U+201C) in UTF-8
		"\xE2\x80\x9D" => '"', // ” (U+201D) in UTF-8
		"\xE2\x80\x9E" => '"', // „ (U+201E) in UTF-8
		"\xE2\x80\x9F" => '"', // ‟ (U+201F) in UTF-8
		"\xE2\x80\xB9" => "'", // ‹ (U+2039) in UTF-8
		"\xE2\x80\xBA" => "'", // › (U+203A) in UTF-8
	);
	$string = strtr($string, $quotes);

	// Version 2
	$search = array(
		chr(145),
		chr(146),
		chr(147),
		chr(148),
		chr(151)
	);
	$replace = array("'","'",'"','"',' - ');
	$string = str_replace($search, $replace, $string);

	// Version 3
	$string = str_replace(
		array('&#8216;','&#8217;','&#8220;','&#8221;'),
		array("'", "'", '"', '"'),
		$string
	);

	// Version 4
	$search = array(
		'&lsquo;', 
		'&rsquo;', 
		'&ldquo;', 
		'&rdquo;', 
		'&mdash;',
		'&ndash;',
	);
	$replace = array("'","'",'"','"',' - ', '-');
	$string = str_replace($search, $replace, $string);

	return $string;
}

A small question: what about duplicating the special codes to use both the uppercase and the lowercase versions? Like "\xE2\x80\x98" and "\xe2\x80\x98". Also, single quotes around these strings seems to work better.