代码如下  

$file_path = '123.txt';
$content = file_get_contents($file_path);
//按换行符把全部内容分隔成数组
$con_array = explode("n", $content);
//替换掉指定行
     
$con_array[12]="123";
//组合回字符串
$con = implode("n", $con_array);
     
//写回文档
file_put_contents($file_path, $con);