A trivial
$num = $num <= 0 ? $num : -$num ;
or, the better solution, IMHO:
$num = -1 * abs($num)
As @VegardLarsen has posted,
the explicit multiplication can be avoided for shortness but I prefer readability over shortness
I suggest to avoid if/else (or equivalent ternary operator) especially if you have to manipulate a number of items (in a loop or using a lambda function), as it will affect performance.
"If the float is a negative, make it a positive."
In order to change the sign of a number you can simply do:
$num = 0 - $num;
or, multiply it by -1, of course :)