这个函数会全部随机替换,注意preg_replace_callback函数的参数

preg_replace_callback ( mixed $pattern , callable $callback , mixed $subject [, int $limit = -1 [, int &$count ]] )

使用方法

$text = "Welcome to [city]. I want [city] to be a random version each time. [city] should not be the same [city] each time.";

$keyword = "[city]";
$values = array("Orlando", "Dallas", "Atlanta", "Detroit", "Tampa", "Miami");

$result = preg_replace_callback('/' . preg_quote($keyword) . '/', 
  function() use ($values){ return $values[array_rand($values)]; }, $text);

注意：以下的函数不适用于特殊字符的替换：

/**
 * @param 原始字符串
 * @param 原始字符
 * @param 替换字符
 * @param 替换次数
 * @return 替换后的字符串
 */
function replace_n_occurrences ($str, $search, $replace, $n) {

    // Get all occurences of $search and their offsets within the string
    $count = preg_match_all('/\b'.preg_quote($search, '/').'\b/', $str, $matches, PREG_OFFSET_CAPTURE);

    // Get string length information so we can account for replacement strings that are of a different length to the search string
    $searchLen = strlen($search);
    $diff = strlen($replace) - $searchLen;
    $offset = 0;

    // Loop $n random matches and replace them, if $n < 1 || $n > $count, replace all matches
    $toReplace = ($n < 1 || $n > $count) ? array_keys($matches[0]) : (array) array_rand($matches[0], $n);
    foreach ($toReplace as $match) {
        $str = substr($str, 0, $matches[0][$match][1] + $offset).$replace.substr($str, $matches[0][$match][1] + $searchLen + $offset);
        $offset += $diff;
    }

    return $str;

}

使用测试方法

$str = 'I like blue, blue is my favorite colour because blue is very nice and blue is pretty';

$search = 'blue';
$replace = 'red';
$replaceCount = 2;

echo $str."<br>";    

echo replace_n_occurrences($str, $search, $replace, $replaceCount);