Javascrip Code In Php Variable

I would like to know if it is possible to insert workable javascript
code into a variable. For example, in php if I say

<? echo $blah; ?> and $blah equals " ?> <script>
top.navi.document.title </script> <?"

I would want the page to act out the code when i tell it to echo the
variable.

If this is not possible, is there a way to set information from
javascript into a php variable?

Also, i would like to know how to use if statements in javascript. For
example, if I have a variable x and i want to see if x equals "test"
and if it does then it caries out some code.