$f1 = file( "file1" );
$f2 = file( "file2" );

$max = count( $f1_array );
$new = array();

for( $i = 0; $i = $max; $i++ ){

$new[] = $f1[$i] . $f2[$i] . "<br />\n";

}

print_r( $new );

$file1 = @file("letters.txt");
$file2 = @file("numbers.txt");

for ($x=1;$x<count($file1);$x++) {
$output[] = $file1[x] . $file2[x];
}

<?php
// bring each file into an array
$f1 = file( "testfile1.txt" );
$f2 = file( "testfile2.txt" );

// get len of each array
$max = count( $f1 );
$chkmax= count($f2);

// use shorter array as maximum value to avoind errors
if($chkmax<$max){$max=$chkmax;}

// create a new array.  You could also open a file to write to instead
$new = array();
$fp=fopen('testfile3.txt','w');

// do the loop properly.
for( $i = 0; $i <= $max; $i++ ){
  // build the new array properly
  $new[$i] = trim($f1[$i]) . trim($f2[$i]) . "\n";
  echo $new[$i] . '<br>';
  // if you are writing to a new file you can do it here if you wanted
  // check to make sure the file opened before writing
  if ($fp)
  {
    // write to the file
    fwrite($fp,$new[$i]);
  }
}
  // Print out the rows of the array using print_r or
  // if you are writing to a new file you could do it here if you wanted to too
  // print_r($new);
if ($fp)
{
    // make sure to close the file if you are writing one 
   fclose($fp);
}

?>