http://www.leaseweblabs.com/2014/05/how-to-use-yield-keyword-php/

test.php

<?php

$f = fopen("data.txt", "r");

fread($f, 10);
fread($f, 10);

// Run with: strace -e trace=read php test2.

/*
read(3, "<?php\nclass File {\n \n  private $"..., 8192) = 8192

*/

test2.php

<?php

$f = fopen("data.txt", "r");
stream_set_chunk_size($f, 5);

fread($f, 10);
fread($f, 10);

// Run with: strace -e trace=read php test2.

/*
read(3, "<?php", 5)                     = 5
read(3, "\nclas", 5)                    = 5
read(3, "s Fil", 5)                     = 5
read(3, "e {\n ", 5)                    = 5

*/