Yes, but how does the Arduino know if you're going to type "test1234" or "test123456789"? How does it know when it can start printing the text on your LCD?
Normally, this is done by sending "test1234" followed by the carriage return character (ASCII 0x0D). Do you know what I mean by the carriage return character? That is very important. It's like "test1234\r" in the C++ language. You could even use "test1234\n" if you wanted and test for the ASCII Newline character (0x0A). Do you know what I mean here? It's like hitting "Enter" on your keyboard. It tells the computer that that is the end of the text line.
Try this: Use my earlier code which has the "0x0D" things in it. In the serial port connection, type the letters "test1234" and hit enter. Don't use your PHP code. Just use a serial port and a communications program, and type the characters in by hand. When you hit "enter", the text should start to scroll. Then when you start typing a new line on your serial port, the text will stop until you hit enter again. Hopefully doing that will help you understand what you need to do.
Or possibly use my "0x0D" code and change your PHP code so that instead of just outputting "test1234" it outputs "test1234\r" (where \r is the C/C++/PHP escape sequence for the ASCII character 0x0D)