fgets()

(PHP 4, PHP 5, PHP 7)

从文件指针中读取一行

说明

从文件指针中读取一行。

参数

文件指针必须是有效的，必须指向由fopen()或fsockopen()成功打开的文件(并还未由fclose()关闭)。

从$handle指向的文件中读取一行并返回长度最多为$length- 1 字节的字符串。碰到换行符（包括在返回值中）、EOF 或者已经读取了 length - 1 字节后停止（看先碰到那一种情况）。如果没有指定$length，则默认为 1K，或者说 1024 字节。

从 PHP 4.3 开始，忽略掉 length 则行的长度被假定为 1024，将继续从流中读取数据直到行结束。如果文件中的大多数行都大于 8KB，则在脚本中指定最大行的长度在利用资源上更为有效。

返回值

从指针$handle指向的文件中读取了$length- 1 字节后返回字符串。如果文件指针中没有更多的数据了则返回FALSE。

错误发生时返回FALSE。

更新日志

范例

逐行读取文件

注释

Note:

习惯了 C 语言中fgets()语法的人应该注意到EOF是怎样被返回的。

参见

• fgetss() 从文件指针中读取一行并过滤掉 HTML 标记
• fread() 读取文件（可安全用于二进制文件）
• fgetc() 从文件指针中读取字符
• stream_get_line() 从资源流里读取一行直到给定的定界符
• fopen() 打开文件或者 URL
• popen() 打开进程文件指针
• fsockopen() 打开一个网络连接或者一个Unix套接字连接
• stream_set_timeout()Set timeout period on a stream

A better example, to illustrate the differences in speed for large files, between fgets and stream_get_line.
This example simulates situations where you are reading potentially very long lines, of an uncertain length (but with a maximum buffer size), from an input source.
As Dade pointed out, the previous example I provided was much to easy to pick apart, and did not adequately highlight the issue I was trying to address.
Note that specifying a definitive end-character for fgets (ie: newline), generally decreases the speed difference reasonably significantly. 
#!/usr/bin/php

$ ./testcase.php 
stream_get_line is faster than fgets - pdiff is 1.760398041785
Note that, in a vast majority of situations in which php is employed, tiny differences in speed between system calls are of negligible importance.

There's an error in the documentation:
The file pointer must be valid, and must point to a file successfully opened by fopen() or fsockopen() (and not yet closed by fclose()).
You should also add "popen" and "pclose" to the documentation. I'm a new PHP developer and went to verify that I could use "fgets" on commands that I used with "popen".

One thing I discovered with fgets, at least with PHP 5.1.6, is that you may have to use an IF statement to avoid your code running rampant (and possibly hanging the server). This can cause problems if you do not have root access on the server on which you are working.
This is the code I have implemented ($F1 is an array):

I have noticed that without the IF statement, fgets seems to ignore when $fh is undefined (i.e., "filename" does not exist). If that happens, it will keep attempting to read from a nonexistent filehandle until the process can be administratively killed or the server hangs, whichever comes first.

The file pointer that fgets() uses can also be created with the proc_open() function and used with the stdout pipe created from the executed process.

I had loads of trouble while reading with fgets on a WAMP (Windows server). On local the file went unto a 
 tag without a hitch, but when I moved the code to a LAMP production server, every \r\n created two fgets and I got free empty lines.
I tried deleting with $string=str_replace("\r\n","\n",$string); but it had no effect whatsoever. The solution was to do an fread() and explode the contents by PHP_EOL and do a foreach($lines as $line) so every line did not get duplicated.
Here is the example code:
$file=fopen("test.txt,"r");
$text=fread($file,filesize("test.txt"));
$lines=explode(PHP_EOL,$text);
foreach($lines as $line)
 {
 // Do something
 }
Macintosh line endings mentioned in docs refer to Mac OS Classic. You don't need this setting for interoperability with unixish OS X.
I think that the quickest way of read a (long) file with the rows in reverse order is

It echos the rows while it is reading the file so it is good for long files like logs.
Borgonovo
It's strange no one mentions "0" in this context.
Since "0" is considered to be false, a line with a single "0" can be treated as EOF if using the while assign idiom.
while ($line = fgets(STDIN, 2)) {
}
This may surprisingly break if a line starts with ")"

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