(PECL eio >= 0.0.1dev)
eio_write — Write to file
$fd,$str,$length = 0,$offset = 0,$pri = EIO_PRI_DEFAULT,$callback = NULL,$data = NULL
eio_write() writes up to length
bytes from str at offset
offset from the beginning of the file.
fdStream, Socket resource, or numeric file descriptor, e.g. returned by eio_open()
strSource string
lengthMaximum number of bytes to write.
offsetOffset from the beginning of file.
pri请求的优先级：EIO_PRI_DEFAULT，EIO_PRI_MIN，EIO_PRI_MAX 或 null。如果是
null，pri 将设为
EIO_PRI_DEFAULT。
callback
callback
函数在请求完成时被调用。其应匹配一下原型：
void callback(mixed $data, int $result[, resource $req]);data传递给请求的用户数据。
result针对请求的结果的值。通常是相应的系统调用返回的值。
req可选的请求资源，可被 eio_get_last_error() 之类的函数使用。
data
Arbitrary variable passed to callback.
eio_write() returns request resource on success, 或者在失败时返回 false.