The function below will show you how to make a search engine optimised slug for a category or post title with the same output that you will get from WordPress' sanitize_title_with_dashes 
function.
All three functions are required.
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<?php
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/*
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Sanitise name for safe URL
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Source: http://codex.wordpress.org/Function_Reference/sanitize_title_with_dashes
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Usage: sanitize_title_with_dashes();
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http://www.beliefmedia.com/code/php-snippets/post-slugs
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*/
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// Preserve escaped octets.
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// Remove percent signs that are not part of an octet.
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// Restore octets.
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}
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}
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if ( 'save' == $context ) {
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// Convert nbsp, ndash and mdash to hyphens
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// Strip these characters entirely
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// iexcl and iquest
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'%c2%a1', '%c2%bf',
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// angle quotes
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'%c2%ab', '%c2%bb', '%e2%80%b9', '%e2%80%ba',
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// curly quotes
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'%e2%80%98', '%e2%80%99', '%e2%80%9c', '%e2%80%9d',
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'%e2%80%9a', '%e2%80%9b', '%e2%80%9e', '%e2%80%9f',
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// copy, reg, deg, hellip and trade
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'%c2%a9', '%c2%ae', '%c2%b0', '%e2%80%a6', '%e2%84%a2',
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// acute accents
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'%c2%b4', '%cb%8a', '%cc%81', '%cd%81',
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// grave accent, macron, caron
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'%cc%80', '%cc%84', '%cc%8c',
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), '', $title );
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// Convert times to x
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}
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return $title;
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}
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<?php
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/*
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Sanitise name for safe URL
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Source: http://codex.wordpress.org/Function_Reference/sanitize_title_with_dashes
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Usage: sanitize_title_with_dashes();
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http://www.beliefmedia.com/code/php-snippets/post-slugs
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*/
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for ($i=0; $i < $length; $i++) {
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if ($c < 0x80) $n = 0; # 0bbbbbbb
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elseif (($c & 0xE0) == 0xC0) $n=1; # 110bbbbb
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elseif (($c & 0xF0) == 0xE0) $n=2; # 1110bbbb
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elseif (($c & 0xF8) == 0xF0) $n=3; # 11110bbb
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elseif (($c & 0xFC) == 0xF8) $n=4; # 111110bb
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elseif (($c & 0xFE) == 0xFC) $n=5; # 1111110b
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else return false; # Does not match any model
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for ($j=0; $j<$n; $j++) { # n bytes matching 10bbbbbb follow ?
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return false;
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}
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}
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return true;
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}
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<?php
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/*
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Sanitise name for safe URL
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Source: http://codex.wordpress.org/Function_Reference/sanitize_title_with_dashes
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Usage: sanitize_title_with_dashes();
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http://www.beliefmedia.com/code/php-snippets/post-slugs
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*/
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$unicode = '';
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$values = array();
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$num_octets = 1;
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$unicode_length = 0;
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for ($i = 0; $i < $string_length; $i++ ) {
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if ( $value < 128 ) {
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if ( $length && ( $unicode_length >= $length ) )
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break;
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$unicode_length++;
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} else {
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$values[] = $value;
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if ( $length && ( $unicode_length + ($num_octets * 3) ) > $length )
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break;
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if ($num_octets == 3) {
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$unicode_length += 9;
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} else {
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$unicode_length += 6;
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}
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$values = array();
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$num_octets = 1;
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}
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}
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}
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return $unicode;
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}
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?>
Usage
Returns: this-is-a-cool-title.
