best way to remove items from a list php - php

Question

I am trying to delete items inside List A from list B
And the two inside a text file
Example : a.txt
1
3
6
b.txt
2
3
6
I tried more than one method previously but with large files it does not work as it should
$a = file('a.txt', FILE_IGNORE_NEW_LINES);
$b = file('b.txt', FILE_IGNORE_NEW_LINES);
$n = 'new.txt';
for ($i = 0;$i < count($b);$i++)
{
if (!in_array($b[$i], $a))
{
$c = file_get_contents($n);
$c .= $b[$i] . "\n";
file_put_contents($n, $c);
}
}
Is there a better way to handle large files like 80k line?

Answer 1

This code mainly changes the way the files are read and written, so that the second file is read 1 line at a time and not read all in memory. The output also uses FILE_APPEND in file_put_contents() so that it doesn't need to read the file again.
The first part is to create an array of the a.txt file, with the value as the index to allow you to use isset() rather than in_array() which will make the searching a lot quicker.
Then read the second file 1 line at a time, check if it's present and add the data if needed...
$fileA = fopen('a.txt', 'r');
$a = [];
while($entry = fgets($fileA))
{
$a[trim($entry)] = true;
}
$fileB = fopen('b.txt', 'r');
$n = 'new.txt';
// Clear the file
file_put_contents($n, '');
while($b = fgets($fileB))
{
if (!isset($a[trim($b)]))
{
file_put_contents($n, $b, FILE_APPEND);
}
}