I tried to do it like 5*log(7.5/2), but I think it's the wrong way, can anyone tell me how to do it right ?
In PHP, a base 10 logarithm is made using log10, not log (which is for natural logarithm i.e. base e). Note also that your comparison example in the comments is with 5.7, not 7.5. Try this instead:
echo 1+5*log10(5.7/2);
Output:
3.2742243000426
Demo on 3v4l.org