Define "ASCII readable format". Do you just want to arbitrarily read each successive group of 8 bits as the equivalent ASCII character that would correlate to that binary number, or something more meaningful (which would require understanding the format/structure of the particular binary file in question). For the former, you could try this completely untested idea:
$hexString = bin2hex($binaryString);
$ascii = '';
for($ix = 0; $ix <= strlen($hexString) - 1; $ix += 2)
{
$ascii .= chr(hexbin(substr($hexString, $ix, 2)));
}
echo $ascii;
Of course, this could result in some unprintable characters since not all ASCII values in the 0-127 range are printable.