See the documentation for number_format: http://php.net/number_format
The functions parameters are:
string number_format ( float $number , int $decimals = 0 , string $dec_point="." , string $thousands_sep = ',' )
So use:
number_format(1000.5, 2, '.', '');
Which means that you don’t use any (= empty string) thousands separator, only a decimal point.