[emilv]

$fh = fopen('indextemplate.html', 'r');
while(!feof($fh)) {
$line = fgets($fh);
if(strpos($line, '{INCLUDE HERE}') !== false) {
$include = file_get_contents('includefile');
$line = str_replace('{INCLUDE HERE}', $include, $line);
}
echo $line;
}
fclose($fh);

eller kortare (men något ineffektivare):

$file = file_get_contents('indextemplate');
$include = file_get_contents('includefile');
$file = str_replace('{INCLUDE HERE}', $include, $file);
echo $file;