Chunk file

chunk-file.php

<?php

// this will convert file lines into a big array. So, if your file has 1000 lines the array count will be 1000
// if your file size 20 MB than this method will occupy 20MB of system ram . 

$lines = file('big_file.txt');

// Now we are going to chunk the lines by 100000 on each chunk
// array_chunk will create a multi demential array with chunked line on array. 
$chunked = array_chunk($lines,100000);

// use chunk
foreach( $chunked as $key => $value ){
	
	if ( is_array($value) ){

		// this must be a array with all chunked lines. count will be 100000 as you've defined it in array_chunk
		// now you can loop this array and write to a file or do whatever.
		foreach( $value as $nkey => $nval ){
			echo $nval;
		}
	}
}