Eclipse and other editors may not correctly store a pound sign (ASCII 163) literal £ in your source code depending on your editor and OS character set.
A safe way to identify the pound sign is to use the PHP chr() function to programatically create the literal for use in comparisons:
$amount = $_POST['amount'];
$pound = chr(163);
$amount = str_replace($pound, '', $amount);
$pound = chr(163);
$amount = str_replace($pound, '', $amount);