<?php
 $fichier = "test.txt";
 $fp = fopen($fichier, "r+");
 $contenu = fread($fp, filesize($fichier)); 
 
  fwrite($fp, $_SERVER['REMOTE_ADDR'] . "\n");
 

 fclose($fp);
?>

essaies avec la constante PHP_EOL

$ip = $_SERVER["REMOTE_ADDR"].PHP_EOL;

$ip = $_SERVER["REMOTE_ADDR"].PHP_EOL;

<?php
$Fnm = 'r1.html';
$inF = fopen($Fnm,"a");
$ptr = ftell($inF);
$ptr = fseek($inF,$ptr-10);
$ip = $_SERVER["REMOTE_ADDR"]."\r\n";
fputs($inF,$ip);
fclose($inF);
?>

<?php
$Fnm = 'r1.html';
$inF = fopen($Fnm,"a");
$ptr = ftell($inF);
$ptr = fseek($inF,$ptr-10);
$ip = $_SERVER["REMOTE_ADDR"]."\n";
fputs($inF,$ip);
fclose($inF);
?>

$chaine .= "\r\n";

\n

 $ip = $_SERVER["REMOTE_ADDR"]."\n";

<?php
$Fnm = 'r1.html'; 
$inF = fopen($Fnm,"a");
$ptr = ftell($inF);
$ptr = fseek($inF,$ptr-10);
$ip = $_SERVER["REMOTE_ADDR"];
fputs($inF,$ip);
fclose($inF);
?>