Php only finds the last row of the second file to be excisting in the array; even if all the rows are the same.

// Open file with 2 columns and parse into array
if ($fd = @fopen("int-osocat.txt", "r")) {
  while (!feof($fd)) {
     $line = fgets($fd, 4096);
     $intercat = strtok($line, "\t");
     $osocat =  strtok("\t");
     $key = $intercat;
     $catvars[$key]["active"] = 1;
     $catvars[$key]["osocat"] = $osocat;
  }
fclose ($fd);
}

// open second file, and check contents against array
if ($fd = @fopen("art-intcat.txt", "r")) {
  while (!feof($fd)) {
     $line = fgets($fd, 4096);
     $artikelnr = strtok($line, "\t");
     $intcat =  strtok("\t");
     $key2 = $intcat;
        if ($catvars[$key2]["active"]== 1) {
          echo $artikelnr . "\t" . $catvars[$key2]["osocat"] . "<br>";
        }
  }
fclose ($fd);
}