Convert a string to a double - is this possible?

Just use floatval().

E.g.:

$var = '122.34343';
$float_value_of_var = floatval($var);
echo $float_value_of_var; // 122.34343

And in case you wonder doubleval() is just an alias for floatval().

And as the other say, in a financial application, float values are critical as these are not precise enough. E.g. adding two floats could result in something like 12.30000000001 and this error could propagate.

For arbitrary precision mathematics PHP offers the Binary Calculator which supports numbers of any size and precision, represented as strings.

$s = '1234.13';
$double = bcadd($s,'0',2);

PHP: bcadd

Use doubleval(). But be very careful about using decimals in financial transactions, and validate that user input very carefully.