sum doesn't work

With this formula (formula 1)

$summa = ($a_100*$p_100_euro+$a_200*$p_200_euro+$a_700*$p_700_euro+$a_701*$p_701_euro+$a_702*$p_702_euro+$a_703*$p_703_euro+$a_704*$p_704_euro+$a_705*$p_705_euro+$a_706*$p_706_euro+$a_707*$p_707_euro+$a_708*$p_708_euro+$a_709*$p_709_euro+$a_800*$p_800_euro+$a_801*$p_801_euro+$a_802*$p_802_euro+$a_803*$p_803_euro+$a_804*$p_804_euro+$a_805*$p_805_euro+$a_806*$p_806_euro+$a_807*$p_807_euro+$a_808*$p_808_euro+$a_809*$p_809_euro+$a_810*$p_810_euro+$a_811*$p_811_euro+$a_812*$p_812_euro+$a_813*$p_813_euro+$a_816*$p_816_euro+$a_817*$p_817_euro+$a_822*$p_822_euro+$a_830*$p_830_euro);

I get a wrong print out. It prints out $summa = 770.

Changing the formula to (formula 2):

$summa = "$a_100*$p_100_euro+$a_200*$p_200_euro+$a_700*$p_700_euro+$a_701*$p_701_euro+$a_702*$p_702_euro+$a_703*$p_703_euro+$a_704*$p_704_euro+$a_705*$p_705_euro+$a_706*$p_706_euro+$a_707*$p_707_euro+$a_708*$p_708_euro+$a_709*$p_709_euro+$a_800*$p_800_euro+$a_801*$p_801_euro+$a_802*$p_802_euro+$a_803*$p_803_euro+$a_804*$p_804_euro+$a_805*$p_805_euro+$a_806*$p_806_euro+$a_807*$p_807_euro+$a_808*$p_808_euro+$a_809*$p_809_euro+$a_810*$p_810_euro+$a_811*$p_811_euro+$a_812*$p_812_euro+$a_813*$p_813_euro+$a_816*$p_816_euro+$a_817*$p_817_euro+$a_822*$p_822_euro+$a_830*$p_830_euro";

it prints out like: 1*1 905+0*1 461+0*100+1*388+0*265+0*109+0*31+0*604+0*137+0*238+0*1 581+0*1 503+0*173+0*311+0*265+1*381+0*355+0*84+0*133+0*249+0*87+0*425+0*485+0*27+0*258+0*391+0*26+0*123+0*53+0*426

If I add it manually together I get 2674. And that is what I expected formula 1 would have given me.

What have I done to cause this miscalculation? It might be a typo, but I can't find it.

Please, gurus, advice me.