In you html page, put an unique string... something like %%PHP_CONTENT%%.
Open the html page and store its content in a variable. After, use eregi_replace() to replace %%PHP_CONTENT%% with what you want to go there...
$replacement_content = "blablabla...";
$fch = "page.html";
$fp = fopen($fch, "r");
$f_content = fread($fp, filesize($fch));
fclose($fp);
$f_content = eregi_replace("%%PHP_CONTENT%%", $replacement_content, $f_content);
echo $f_content;
%%PHP_CONTENT%%, in this exemple, will be replaced by blablabla...