Using ssh_exec, one could execute only one command at a time. However, ssh_shell, offers greater flexibility.
Here is an example:
<?php
$con=ssh2_connect('192.168.0.1', 22);
ssh2_auth_password($con, "user", "password");
$shell=ssh2_shell($con, 'xterm');
fwrite( $shell, "cd /dir_one\n");
fwrite( $shell, "ls -la\n");
fwrite( $shell, "cd /dir_two\n");
fwrite( $shell, "ls -la\n");
?>
Thanks Sara for your implementation and your help !