(PHP 4, PHP 5, PHP 7, PHP 8)
fseek — 在文件指针中定位
$stream, int $offset, int $whence = SEEK_SET): int
在与 stream 关联的文件中设定文件指针位置。新位置从文件头开始以字节数度量，是以
whence 指定的位置加上 offset。
In general, it is allowed to seek past the end-of-file; if data is then written, reads in any unwritten region between the end-of-file and the sought position will yield bytes with value 0. However, certain streams may not support this behavior, especially when they have an underlying fixed size storage.
stream文件系统指针，是典型地由 fopen() 创建的 resource(资源)。
offset偏移量。
要移动到文件尾之前的位置，需要给 offset 传递一个负值，并设置
whence 为 SEEK_END。
whence
whence 值是：
SEEK_SET - 设定位置等于 offset 字节。SEEK_CUR - 设定位置为当前位置加上 offset。SEEK_END - 设定位置为文件尾加上 offset。成功则返回 0；否则返回 -1。
示例 #1 fseek() 例子
<?php
$fp = fopen('somefile.txt', 'r');
// read some data
$data = fgets($fp, 4096);
// move back to the beginning of the file
// same as rewind($fp);
fseek($fp, 0);
?>
注意:
如果使用附加模试（
a或a+），任何写入文件数据都会被附加上去，而文件的位置将会被忽略，调用 fseek() 的结果尚未定义。
注意:
Not all streams support seeking. For those that do not support seeking, forward seeking from the current position is accomplished by reading and discarding data; other forms of seeking will fail.