Note: string can be as large as 2GB.
file_get_contents如果正常返回，会把文件内容储存到某个字符串中，所以它不应该返回超过2G长度的字符串。
如果文件内容超过2G，不加offset和maxlen调用file_get_contents的话，肯定会返回false，
在64位机上面啥错都不报啊，我晕:

PHP 5.4.16 (cli) (built: Jun 7 2013 14:32:19)
Copyright (c) 1997-2013 The PHP Group
Zend Engine v2.4.0, Copyright (c) 1998-2013 Zend Technologies
with XCache v3.0.2, Copyright (c) 2005-2013, by mOo
with XCache Optimizer v3.0.2, Copyright (c) 2005-2013, by mOo
with XCache Cacher v3.0.2, Copyright (c) 2005-2013, by mOo
with XCache Coverager v3.0.2, Copyright (c) 2005-2013, by mOo
 

回到32位机，报这样的错：
PHP 5.3.10 (cli) (built: Jul 7 2012 12:30:39)
Copyright (c) 1997-2012 The PHP Group
Zend Engine v2.3.0, Copyright (c) 1998-2012 Zend Technologies

Warning: file_get_contents(./2g): failed to open stream: File too large in /data/app/zhaokunyao/test.php on line 4

/* {{{ proto string file_get_contents(string filename [, bool use_include_path [, resource context [, long offset [, long maxlen]]]])
Read the entire file into a string */
PHP_FUNCTION(file_get_contents)
{
char *filename;
int filename_len;
char *contents;
zend_bool use_include_path = 0;
php_stream *stream;
#int 的最大值是 2^31 -1 = 2147483648 -1 = 2G -1
int len;
long offset = -1;
long maxlen = PHP_STREAM_COPY_ALL;
zval *zcontext = NULL;
php_stream_context *context = NULL;

/* Parse arguments */
if (zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "p|br!ll", &filename, &filename_len, &use_include_path, &zcontext, &offset, &maxlen) == FAILURE) {
return;
}

if (ZEND_NUM_ARGS() == 5 && maxlen < 0) {
php_error_docref(NULL TSRMLS_CC, E_WARNING, "length must be greater than or equal to zero");
RETURN_FALSE;
}

context = php_stream_context_from_zval(zcontext, 0);

stream = php_stream_open_wrapper_ex(filename, "rb",
(use_include_path ? USE_PATH : 0) | REPORT_ERRORS,
NULL, context);
if (!stream) {
RETURN_FALSE;
}

if (offset > 0 && php_stream_seek(stream, offset, SEEK_SET) < 0) {
php_error_docref(NULL TSRMLS_CC, E_WARNING, "Failed to seek to position %ld in the stream", offset);
php_stream_close(stream);
RETURN_FALSE;
}

#php_stream_copy_to_mem的返回值是 size_t ， 32位机上是 unsigned int ， 64位机上是 long unsigned int.
#如果php_stream_copy_to_mem的返回值大于2G，那么len会溢出成为负数 <0，所以会返回FALSE.
#其实也有可能溢出成为正数的，比如4294968320...就会变成1024。。于是file_get_contents() 只返回前1024字节。

if ((len = php_stream_copy_to_mem(stream, &contents, maxlen, 0)) > 0) {
RETVAL_STRINGL(contents, len, 0);
} else if (len == 0) {
RETVAL_EMPTY_STRING();
} else {
RETVAL_FALSE;
}

php_stream_close(stream);
}