$read = file("f.txt");
<div id="d">+</div>
<div><?php echo $read[0]; ?></div>
1
<?php
$count = file_get_contents("f.txt");
echo "<div id='d'><a href='/increase.php'>+</a></div><div>{$count}</div>";
?>
<?php
$count = intval(file_get_contents("f.txt"));
$count++;
file_put_contents("f.txt",$count);
header("Location: index.php");
?>