<?php
echo 'a\a';
echo "\n";
echo 'a\\a';
echo "\n";
echo 'a\\\a';
echo "\n";
echo 'a\\\\a';
echo "\n";
echo 'a\\\\\a';
echo "\n";
echo 'a\\\\\\a';

a\a
a\a
a\\a
a\\a
a\\\a
a\\\a