Description:
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(1) Open a COM port as a serial input using fopen. Read using fread or fgets. No data appears until 8K bytes have been read. There is an 8K buffer on writes, but there shouldn't be one on reads. (Should there?) Are reads somehow using the write buffer? Sure enough:

(2) Again, open a COM port as a serial input using fopen. This time, write some characters using fwrite. Now fread 8K of serial data. At the head of that stream appears the data you had written with fwrite. Oops.

[Several people have reported an inability to get fgets to read COM data, saying they tried and fgets just never sees any input. I assume this was their problem. But others have reported using fgets on COM successfully. Something specific to OS/PHP versions?] 

Reproduce code:
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(1) 
$serial_port = fopen("COM4", "r+");
echo(fread($serial_port, 8192));

(2)
$serial_port = fopen("COM4", "r+");
fwrite($serial_port, "MP");
echo(fread($serial_port, 8192));

Expected result:
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(1) & (2)
serial data starting with the first byte that came in on the serial line


Actual result:
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(1) Nothing until 8K of data has been read in.

(2) After enough serial data to fill the buffer, first the data written ("MP" in this example), then the data that came in on the serial line.