Unable to replace a variable value

[j_limboo]

<?php
$ser = $_GET['q'];
$logic = "~resume (filetype:pdf OR filetype:doc OR filetype:rtf OR filetype:htm OR filetype:html) skill -jobs -apply -submit -required -wanted -write -sample";
str_replace ("skill", "$ser", $logic);
echo $logic;

?>

I am not able to replace skill please help

New to PHP

Please advice
Thank You

[smartness]

<?php
$ser = $_GET;
$logic = "~resume (filetype:pdf OR filetype:doc OR filetype:rtf OR filetype:htm OR filetype:html) skill -jobs -apply -submit -required -wanted -write -sample";
$logic = str_replace ("skill", "$ser", $logic);
echo $logic;

?>

Here

[j_limboo]

Smartness, you are life saver thank you for making this happen to a baby learner

[smartness]

Your welcome... I'm a beginner tooo. p.s. click solved or change the title to [Solved] + Title...