The script :
-------------------
$nom="mail";
$adr=gethostbyname($nom);
echo "$nom=$adr<BR />";
$fp=fsockopen ($adr, 110, $errno, $errstr, 30);
if (!$fp) {
    echo "$errstr ($errno)<br>\n";
} else {
    echo fgets ($fp,128) . "<BR />\n";
    fputs ($fp, "QUIT\r\n");
    echo fgets ($fp,128) . "<BR />\n";
    fclose ($fp);
    }

 ?>
----- Result (work's fine) ----------
mail=172.18.1.1
+OK POP3 iut-info v4.41 server ready
+OK Sayonara 
------ But if i try (other lines not changed) ------
$fp=fsockopen ($nom, 110, $errno, $errstr, 30);
----- Result ------------
mail=172.18.1.1

Warning: fsockopen() [function.fsockopen]: php_network_getaddresses: getaddrinfo failed: host nor service provided, or not known in /home/prof/collet/public_html/testsock.php on line 14

Warning: fsockopen() [function.fsockopen]: unable to connect to mail:110 in /home/prof/collet/public_html/testsock.php on line 14
Protocol not supported (221)
-----------------------------------------------
From the doc, hostname could be a name, not only a IP.

----- PHP Compile with ---------------
./configure                             \
        --with-oci8                     \
        --with-apache=../apache_1.3.27  \
        --with-gd                       \
        --with-pdflib=/opt/pdflib       \
        --with-jpeg-dir                 \
        --with-png-dir                  \
        --with-tiff-dir                 \
        --with-zlib                     \
        --with-bz2                      \
        --enable-sigchild               \
        --with-mysql=/opt/mysql         \
        --with-pgsql=/opt/pgsql         \
        --with-tsrm-pthreads            \
        --with-dom                      \
        --enable-sockets
------------------------------
Cordialy.