Prova questo.

Codice PHP:
<?php
$descrizione = file_get_contents("pippo.txt");
$descrizione = str_replace("\n"," ",$descrizione);
$dbtext      = str_replace("\r","",$descrizione);
$db_text     = wordwrap($dbtext, 15, "\r\n");  
file_put_contents('pippo_2.txt',$db_text);
print "Fatto..........";
?>
Prima : pippo.txt
codice:
AAAAAAAAAAAAA AAAAAA AAAAAA AAAAAAAAA AAAAAAAA AAAAAAAAAA AAAAAA
BBBBBBBBBBBBB BBBBBB BBBBBB BBBBBBBBB BBBBBBBB BBBBBBBBBB BBBBBB
CCCCCCCCCCCCC CCCCCC CCCCCC CCCCCCCCC CCCCCCCC CCCCCCCCCC CCCCCC
DDDDDDDDDDDDD DDDDDD DDDDDD DDDDDDDDD DDDDDDDD DDDDDDDDDD DDDDDD
EEEEEEEEEEEEE EEEEEE EEEEEE EEEEEEEEE EEEEEEEE EEEEEEEEEE EEEEEE
Dopo : pippo_2.txt
codice:
AAAAAAAAAAAAA
AAAAAA AAAAAA
AAAAAAAAA
AAAAAAAA
AAAAAAAAAA
AAAAAA
BBBBBBBBBBBBB
BBBBBB BBBBBB
BBBBBBBBB
BBBBBBBB
BBBBBBBBBB
BBBBBB
CCCCCCCCCCCCC
CCCCCC CCCCCC
CCCCCCCCC
CCCCCCCC
CCCCCCCCCC
CCCCCC
DDDDDDDDDDDDD
DDDDDD DDDDDD
DDDDDDDDD
DDDDDDDD
DDDDDDDDDD
DDDDDD
EEEEEEEEEEEEE
EEEEEE EEEEEE
EEEEEEEEE
EEEEEEEE
EEEEEEEEEE
EEEEEE