Deeply understand the difference between ob_flush and flush in php

ob_start();
echo '1';
ob_flush();//Output the php cache and refresh it
echo '2';
ob_flush();//Output the php cache and refresh it
$cc = ob_get_contents();
ob_end_clean();
var_dump($cc);
?>

output: 12string(0) ""

Test 2:

ob_start();
echo '1';
flush(); //Output apache cache and refresh it
echo '2';
flush(); //Output apache cache and Refresh
$cc = ob_get_contents();
ob_end_clean();
var_dump($cc);
?>

output: string(2) "12"

Test 3:

ob_start();
echo '1';
ob_flush();//Output php cache and refresh
flush();//Output apache cache and refresh
echo '2' ;
ob_flush();//Output php cache and refresh
flush(); //Output apache cache and refresh
$cc = ob_get_contents();
ob_end_clean();
var_dump($cc);
?>

Output: 12string(0) ""

Test 4,

ob_start();
echo '1';
flush();//Output apache cache and refresh
ob_flush();//Output php cache and refresh
echo '2' ;
flush();//Output apache cache and refresh
ob_flush(); //Output php cache and refresh
$cc = ob_get_contents();
ob_end_clean();
var_dump($cc);
?>

Output: 12string(0) ""