Works just fine here.

Ok, and strpos() doesn't find any \n on the search string although there are surely \n in there.
I've just seen that was another bug. So this bug is surely realted, but please fix that "undefined function ()" in "if ($a || $b)" at last!!

Have you replaced...

$lfPos = strpos($this->magicBuffer, "\n");

....with...

$lfPos = strpos($this->magicBuffer, "\r");

...?

# php /tmp/2.php
strpos() has been successfuly run!

I'm sure you don't need a class to check if strpos() finds '\r' or '\n' in strings.

Thank you for this bug report. To properly diagnose the problem, we
need a short but complete example script to be able to reproduce
this bug ourselves. 

A proper reproducing script starts with <?php and ends with ?>,
is max. 10-20 lines long and does not require any external 
resources such as databases, etc.

If possible, make the script source available online and provide
an URL to it here. Try to avoid embedding huge scripts into the report.

Bug update collision, setting back to bogus.

Ok, changed to PHP/5.1.4-1.dotdeb.1. Still the error.
CLI as libapache2 produce the same error.

$ php BUG.php 
strpos() has been successfuly run!
Fatal error: Call to undefined function  () in /home/www/Projekte/flyerpilot.de/htdocs.dev/MimeMagic/BUG.php on line 41

OK COPY AND PASTE FOLLWING SCRIPT AND TRY AGAIN!
--------------------------------------------------
<?php

class MagicReader {
	
	private $magicBuffer = '';
	private $magicFile;
	
	private $fp;
	
	
	function __construct($filename=null) {
		if (is_string($filename))
			$this->openMagicFile($filename);
		$this->convert();	
		$this->closeMagicFile();
	}
	
	
	function openMagicFile($filename) 	{
		$this->magicFile = @fopen($filename, 'r');
		return $this->magicFile;
	}
	
	
	function closeMagicFile() 	{
		return @fclose($this->magicFile);
	}
	
	
	function convert() {
		while ($line = $this->readLine()) {
			echo $line;
			die();
		}
	}
	
	
	private function readLine() 
	{
		
		while ((!@feof($this->magicFile)) || (strlen($this->magicBuffer)>0)) {	
			$lfPos = strpos($this->magicBuffer, "\r"); // cahnge this value to R!
			if ($lfPos !==false) {
				$this->magicBuffer = substr($this->magicBuffer, $lfPos);
				return substr($this->magicBuffer, $lfPos)."|";
			}
			$this->magicBuffer = @fread($this->magicFile, 4096);
		}
		
		// if we reached this position we are at the end of the file!
		
		$out = $this->magicBuffer; // assign return output
		
		if (empty($this->magicBuffer)) {// if buffer is already empty we are really at the end
			return false;
		}
			
		$this->magicBuffer = ''; // clear out buffer
		
		return $out;	
	} 

	
	
	function __destruct() {
		$this->closeMagicFile();
	}
}

$str = "That's a\rlittle Test!";
echo (strpos($str, "\r")) ? "strpos() has been successfuly run!" : "nothing found but successfuly run!";

$reader = new MagicReader("magic");


?>

And copy /usr/share/misc/magic to your working dir so the script find a file! (Otherweise the script won't exit in with PHPCLI)

Works _perfectly fine_.
Please either provide a short reproduce script (do you really need those classes to read a file, eh?) or just debug your code yourself.