You can use double quoted strings, variables in scope will be replaced automatically.

See here for more details: www.php.net

$string = "My string";
$needle = "string";
$replace = "replaced var";
echo str_replace($needle,$replace,$string)

Is this what you want? Or this:
$var = 'special string';
echo "I'm echoing a $var";// Output: I'm echoing a special string

Need more info. As stated is pretty vague.

Need more information.

I assume your doing something more complicated than:

$var = 'paul';

echo 'this person is ill today: ' . $var;