PHP Replace text in file with synonym from dB

I am trying to PHP replace ANY & ALL word and phrase matches in a text file with associated synonyms from a dB and I had it doing single replaces ok but now I want multiple replaces I can't get it to replace at all :(

In the dB is a table with two fields per record:

seed         :  This is the match element to compare against and find occurences of in the text file.
synonym  :  This is a the partner field of the row conmtaining possible synonyms divided by a pipe | (random selection required)

example seed                 :   "100%"
example replacement     :  "100 per cent"
example field data text   :  "100 percent|one hundred pc|100pc"

The text file with the text to search and replace over is called doc.txt

My code (not working) so far is:

mysql_connect("localhost","wuser","p!as1@h");
mysql_select_db("words");

		$str=implode("\n",file('doc.txt'));
		$fp=fopen('doc.txt','w');

		$results = mysql_query("select * from synonyms");
		while ($row = mysql_fetch_assoc($results)) {

        $seed = $row[seed];
        $synonym = $row[synonym];

		$words = explode("|", $synonym);
		$index = rand(0, count($words)-1);
		$choice = $words[$index];

		while(strpos($str, '/$seed/') !== false) {
     	echo "Replacing $seed with $choice<br>";

		$str = str_replace($seed, $choice,$str);
		}
	}
    fwrite($fp,$str,strlen($str));

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Could someone help this novice and show me what I need to do to make it check every line of the doc to see if there is any matches that should be replaced by synonyms?

Thanks