非常简单的实现方法，思路是将目标字符串提取分割开，重新组合合并

$myString = '<p>I like blue, </p><p>blue is my favorite</p> <p>colour because blue is very nice and blue is pretty</p>';
$myVar = '123<p>';
$words = explode('<p>', $myString);      // split the string into words    
$index = rand(0, count($words) - 1);   // select a random index    
$words[$index] = $myVar;               // replace the word at the random position
//$words[$index] = $myVar . $words[$index]; // insert the word at the random position
$myString = implode('<p>', $words);      // merge the string back together from the words 
echo $myString;

或者高级一点替换数组

$myString = '<p>I like blue, </p><p>blue is my favorite</p> <p>colour because blue is very nice and blue is pretty</p>';
$words = explode('<p>', $myString);
$inserted = array( '123</p>' ); // not necessarily an array, see manual quote
array_splice($words, array_rand($words), 0, $inserted ); // splice in at position 3
$myString = implode("<p>", $words);
echo $myString;

如果要使用起来更方便可以封装一下变成这样，参数依次是：原始字符，查找字符，替换字符，起始位置

function insert_random_text($str, $search, $replace, $start=0){
    $words = explode($search, $str);      // split the string into words    
    $index = rand($start, count($words) - 1);   // select a random index    
    $words[$index] = $replace;               // replace the word at the random position  #option1 替换字符
    //$words[$index] = $myVar . $words[$index]; // insert the word at the random position #option2 插入字符,注意插入目标搜索字符串的前或后位置可调整
    $body = implode($search, $words);      // merge the string back together from the words
    return $body;
}                                

另外一个实现随机替换字符串的函数，参数分别是：字符串，目标字符，替换字符，替换次数，不过函数无法实现特殊字符的筛选

/**
 * @param 原始字符串
 * @param 目标字符
 * @param 替换字符
 * @param 替换次数
 * @return 字符串 
 */
    function replace_n_occurences ($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;

    }

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

    $search = 'blue';
    $replace = 'red';
    $replaceCount = 1;

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