Good time of day.
Input

group.csv:

ID; name;
1; group 1;
2; group 2;
3; group 3;
4; Group 4;

Products.csv:

id; category; name; price;
1; 1; commodity1; 10
2; 2; commodity2; 20
3; 3; product3; 30
4; 4; product4; 40

I connected them ran through the cycle

& lt;? php
$ file1 = fopen ("groups.csv", "r");
$ file2 = fopen ("Products.csv", "R");
$ File = Array ($ File1, $ File2);
  Foreach ($ Files AS $ File) {
  While (($ Line = Fgets ($ File))! == False) {
  List ($ Index, $ Value) = Explode (';', Trim ($ Line));
  Echo $ Line;
  echo '& lt; hr & gt; & lt; br & gt;';
  }
}
? & gt;

I can not understand how to make me so that the product substituted the product and the price.

Answer 1, Authority 100%

If I understand correctly, you need to withdraw all products with the correct name of categories.
If so, then:

& lt;? php
// For a start forming an array of categories.
$ filegroups = fopen ("groups.csv", "r");
$ groups = [];
While (($ Line = Fgets ($ filegroups))! == False) {
  $ Data = Explode (';', Trim ($ Line));
  if (! (int) $ Data [0]) Continue; // This condition helps skip the header from the file.
  $ Groups [$ Data [0]] = $ Data [1];
}
FClose ($ filegroups);
// TERRER IN $ GROUPS We have all categories from the file
// Headers from files We ignore and remove your own.
echo 'id; category; product; price of goods;';
echo '& lt; hr & gt; & lt; br & gt;';
// start receiving goods from the file and display them
$ fileProducts = Fopen ("Products.csv", "R");
While (($ Line = Fgets ($ fileproducts))! == FALSE) {
  $ Data = Explode (';', Trim ($ Line));
  if (! (int) $ Data [0]) Continue; // This condition helps skip the header from the file.
  // We form the data array desired to display
  $ Print = [
    $ DATA [0],
    Isset ($ Groups [$ Data [1]])? $ Groups [$ Data [1]]: 'Unknown Category', // We substitute the category if it is.
    $ DATA [2],
    $ DATA [3],
  ];
  // Display in the format we need
  Echo implode (';', $ Print);
  echo '& lt; hr & gt; & lt; br & gt;';
}
FClose ($ fileproducts);

appropriately I left the same as you had. To form a beautiful table, you can use the answer from Bloom

Answer 2, Authority 100%

& lt;? php
$ file1 = fopen ("groups.csv", "r");
$ file2 = fopen ("Products.csv", "R");
$ File = Array ($ File1, $ File2);
  Foreach ($ Files AS $ File) {
  While (($ Line = Fgets ($ File))! == False) {
  $ Value [] = Explode (';', Trim ($ Line));
 }
}
For ($ i = 0; $ i & lt; Count ($ value); $ i ++) {
 Echo '& lt; Table CellSpacing = "0" Border = "2" style = "table-layout: fixed; width: 200px" & gt;';
 Echo '& lt; TR & GT;';
 echo '& lt; TD style = "width: 200px; overflow: hidden; text-overflow: ellipsis; White-space: nowrap" & gt;' $ Value [$ i] [0]. '& lt; TD style = "Width : 200px; "& gt; 'iconv (' CP1251 ',' UTF-8 ', $ Value [$ i] [1]).' & Lt; TD style =" width: 100px; "& gt;" iconv (' CP1251 ',' UTF-8 ', $ Value [$ i] [2]).' & Lt; td style = "width: 200px;" & gt; "iconv ('CP1251', 'UTF-8', $ Value [$ i] [3]). '& lt; td style = "width: 200px;" & gt; "iconv (' CP1251 ',' UTF-8 ', $ Value [$ i] [4]).' & lt ; / td & gt; ';
Echo '& lt; / tr & gt;';
Echo '& lt; / Table & gt;';
}
? & gt;