Do it the other way around:
$a = "1,435";$b = str_replace( ',', '', $a );if( is_numeric( $b ) ) { $a = $b;}
Not tested, but probably something like if(preg_match("/^[0-9,]+$/", $a)) $a = str_replace(...)
if(preg_match("/^[0-9,]+$/", $a)) $a = str_replace(...)
The easiest would be:
$var = intval(preg_replace('/[^\d.]/', '', $var));
or if you need float:
$var = floatval(preg_replace('/[^\d.]/', '', $var));