<?php
/**
 * Parses a String of Tags
 *
 * Tags are space delimited. Either single or double quotes mark a phrase.
 * Odd quotes will cause everything on their right to reflect as one single
 * tag or phrase. All white-space within a phrase is converted to single
 * space characters. Quotes burried within tags are ignored! Duplicate tags
 * are ignored, even duplicate phrases that are equivalent.
 *
 * Returns an array of tags.
 */
function ParseTagString($sTagString)
{
	$arTags = array();		// Array of Output
	$cPhraseQuote = null;	// Record of the quote that opened the current phrase
	$sPhrase = null;		// Temp storage for the current phrase we are building
 
	// Define some constants
	static $sTokens = " 
\t";	// Space, Return, Newline, Tab
	static $sQuotes = "'\"";		// Single and Double Quotes
 
	// Start the State Machine
	do
	{
		// Get the next token, which may be the first
		$sToken = isset($sToken)? strtok($sTokens) : strtok($sTagString, $sTokens);
 
		// Are there more tokens?
		if ($sToken === false)
		{
			// Ensure that the last phrase is marked as ended
			$cPhraseQuote = null;
		}
		else
		{		
			// Are we within a phrase or not?
			if ($cPhraseQuote !== null)
			{
				// Will the current token end the phrase?
				if (substr($sToken, -1, 1) === $cPhraseQuote)
				{
					// Trim the last character and add to the current phrase, with a single leading space if necessary
					if (strlen($sToken) > 1) $sPhrase .= ((strlen($sPhrase) > 0)? ' ' : null) . substr($sToken, 0, -1);
					$cPhraseQuote = null;
				}
				else
				{
					// If not, add the token to the phrase, with a single leading space if necessary
					$sPhrase .= ((strlen($sPhrase) > 0)? ' ' : null) . $sToken;
				}
			}
			else
			{
				// Will the current token start a phrase?
				if (strpos($sQuotes, $sToken[0]) !== false)
				{
					// Will the current token end the phrase?
					if ((strlen($sToken) > 1) && ($sToken[0] === substr($sToken, -1, 1)))
					{
						// The current token begins AND ends the phrase, trim the quotes
						$sPhrase = substr($sToken, 1, -1);
					}
					else
					{
						// Remove the leading quote
						$sPhrase = substr($sToken, 1);
						$cPhraseQuote = $sToken[0];
					}
				}
				else
					$sPhrase = $sToken;
			}
		}
 
		// If, at this point, we are not within a phrase, the prepared phrase is complete and can be added to the array
		if (($cPhraseQuote === null) && ($sPhrase != null))
		{
			$sPhrase = strtolower($sPhrase);
			if (!in_array($sPhrase, $arTags)) $arTags[] = $sPhrase;
			$sPhrase = null;
		}
	}
	while ($sToken !== false);	// Stop when we receive FALSE from strtok()
	return $arTags;
}