<?php

$content = '\tThis variable is not set by me.\nCannot do anything about it.\n';



echo "input:\n======\n$content\n\noutput:\n=======\n",

    preg_replace(
        '/\\\\([nrtvf\\\\$"]|[0-7]{1,3}|\x[0-9A-Fa-f]{1,2})/e',
        'stripcslashes("$0")', $content
    )

;

input:
======
\tThis variable is not set by me.\nCannot do anything about it.\n

output:
=======
	This variable is not set by me.
Cannot do anything about it.