How to do repeated search & replacements:

<?PHP
// This is template_reader.php

$var = "hello, world!";

$marker = "<!--{{PLACEHOLDER}}-->";

$str = file_get_contents("template.tpl");
$str = str_replace($marker,"one...".$marker,$str);
$str = str_replace($marker,"two...".$marker,$str);
$str = str_replace($marker,"three...".$marker,$str);
$str = str_replace($marker,$var,$str);

echo $str;
?>

--- The template:
<html><body><p><!--{{PLACEHOLDER}}--></p></body></html>

--- This is the output:
<html><body><p>one...two...three...hello, world!</p></body></html>