[Help] post data into another php function?

<form action="search.php" method="post">
<input type="text" name="searchterm" />
<input type="submit" />
</form>

<?php $Amazon=new Amazon();  $parameters=array( "region"=>"com", 
"AssociateTag"=>'affiliateTag', 'ResponseGroup'=>'Images',  "Operation"=>"ItemSearch",
 "SearchIndex"=>"Books",  "Keywords"=>"harry potter" );  
$queryUrl=$Amazon->getSignedUrl($parameters);  echo $queryUrl; ?>

 "keywords"=>"$_POST["searchterm"]

 <?php $Amazon=new Amazon();  $parameters=array( "region"=>"com", 
"AssociateTag"=>'affiliateTag', 'ResponseGroup'=>'Images',  "Operation"=>"ItemSearch",
 "SearchIndex"=>"Books",   "keywords"=>$_POST["searchterm"]   );  
$queryUrl=$Amazon->getSignedUrl($parameters);  echo $queryUrl; ?>

<?php class Amazon {      // public key     var $publicKey = "xxxxxxxxxxxxxx";     // private key     var $privateKey = "xxxxxxxxxxxxx";     // affiliate tag
     var $affiliateTag='affiliateTag';          /**         *Get a signed URL         *@param string $region used to define country      
   *@param array $param used to build url         *@return array $signature returns the signed string and its components         */  
   public function generateSignature($param)     {         // url basics         $signature['method']='GET';         $signature['host']='ecs.amazonaws.'.$param['region'];         $signature['uri']='/onca/xml';          // necessary parameters         $param['Service'] = "AWSECommerceService"; 
        $param['AWSAccessKeyId'] = $this->publicKey;         $param['Timestamp'] = gmdate("Y-m-d\TH:i:s\Z");         $param['Version'] = '2009-10-01';         ksort($param);         foreach ($param as $key=>$value)         {             $key = str_replace("%7E", "~", rawurlencode($key));          
   $value = str_replace("%7E", "~", rawurlencode($value));             $queryParamsUrl[] = $key."=".$value;         }   
     // glue all the  "params=value"'s with an ampersand         $signature['queryUrl']= implode("&", $queryParamsUrl);          // we'll use this string to make the signature         $StringToSign = $signature['method']."\n".$signature['host']."\n".$signature['uri']."\n".$signature['queryUrl'];  
       // make signature         $signature['string'] = str_replace("%7E", "~",             rawurlencode(                 base64_encode(                     hash_hmac("sha256",$StringToSign,$this->privateKey,True                     )                 )             )         );         return $signature;     }         /**         * Get signed url response         * @param string $region         * @param array $params         * @return string $signedUrl a query url with signature         */     public function getSignedUrl($params)     {         $signature=$this->generateSignature($params);          return $signedUrl= "http://".$signature['host'].$signature['uri'].'?'.$signature['queryUrl'].'&Signature='.$signature['string'];     } } ?>