<?
  $var="this is a test email";
  echo $var;
   echo "</br>";
  echo strlen($var);
  echo "</br>";
  echo substr_replace($var,"rgb",2);
    echo "</br>";
     echo strlen(substr_replace($var,"rgb",2));
   echo "</br>";
  echo substr_replace($var,"rgb",2,1);
    echo "</br>";
 echo strlen(substr_replace($var,"rgb",2,1));
 echo "</br>";
  echo substr_replace($var,"rgb",2,3);
 echo "</br>";
   echo strlen(substr_replace($var,"rgb",2,3));
?>


请问一下。。substr_replace中的第四个参数是什么意思呢？？

这是需要替换的length！
对这种函数使用的问题，最好是看看技术文档，解释的很详细！