I know this is an old post, but I really hate that there is only one answer here that suggests not mixing html and php. Instead of mixing content one should use template systems, or create a basic template system themselves.
In the php
<?php
$var1 = 'Alice'; $var2 = 'apples'; $var3 = 'lunch'; $var4 = 'Bob';
if ($var1 == 'Alice') {
$html = file_get_contents('/path/to/file.html'); //get the html template
$template_placeholders = array('##variable1##', '##variable2##', '##variable3##', '##variable4##'); // variable placeholders inside the template
$template_replace_variables = array($var1, $var2, $var3, $var4); // the variables to pass to the template
$html_output = str_replace($template_placeholders, $template_replace_variables, $html); // replace the placeholders with the actual variable values.
}
echo $html_output;
?>
In the html (/path/to/file.html)
<p>##variable1## ate ##variable2## for ##variable3## with ##variable4##.</p>
The output of this would be:
Alice ate apples for lunch with Bob.