Description:
------------
ext/standard/file.c#560

    560 	/* uses mmap if possible */
    561 	if ((len = php_stream_copy_to_mem(stream, &contents, maxlen, 0)) > 0) {

main/php_streams.h

    431 /* read all data from stream and put into a buffer. Caller must free buffer when done.
    432  * The copy will use mmap if available. */
    433 PHPAPI size_t _php_stream_copy_to_mem(php_stream *src, char **buf, size_t maxlen,
    434 		int persistent STREAMS_DC TSRMLS_DC);
    435 #define php_stream_copy_to_mem(src, buf, maxlen, persistent) _php_stream_copy_to_mem((src), (buf), (maxlen), (persistent) STREAMS_CC TSRMLS_CC)

main/php_streams.h

    419 /* copy up to maxlen bytes from src to dest.  If maxlen is PHP_STREAM_COPY_ALL, copy until eof(src).
    420  * Uses mmap if the src is a plain file and at offset 0 */
    421 #define PHP_STREAM_COPY_ALL		((size_t)-1)

In this last case, PHP_STREAM_COPY_ALL is also passed to php_stream_copy_to_stream_ex, which still uses mmap+mmap_unmap, so it's half correct (except for the part is works with any maxlen, not just PHP_STREAM_COPY_ALL).