data type casting to numbers with intval() or doubleval()

I am having a problem with the intval() function

I am using it to take string data and extract the number it would
represent if only numeric.

$my_integer = intval($my_string)

when the sting is numbers followed by letters -
$my_string = 4444mmm
$my_integer = 4444
it works as expected

but when letters are followed by numbers
$my_string = mmm4444
$my_integer = 0

Same problem with doubleval()

What am I missing???? how do I drop the non-numeric parts of a
string?

imago