Fifo Method Calculation php

[Billz]

Please help me let me explain with this.....
```
purchase table
id   purchase_id  product_id   qty   net_unit_cost  created_at
1          1         1          10      10            2022-10-10
2          2         1          20      12            2022-10-10

Sale table

sale_id     product_id      qty      net_unit_price    created_at
1               1           11          15             2022-10-10
```
in this, if i sold '11' units then how can i subtract from the rows to get remaining units? i've to subtract '10' units from first row and '1' unit from second row.
Please help how to calculate fifo in php