[PHP] Making a string from an Array

[Sam]

Hi all, 
this may be an easy question, but I'm stuck...

how can this,

$words = explode(" ", $keywordText);

for ( $i = 0; $i < count( $words ); $i++ )
{
 if ($words[$i]=="on" OR $words[$i]=="the" OR $words[$i]=="in"
OR$words[$i]=="at" OR $words[$i]=="is" OR $words[$i]=="it")
 {
  $ignored[] = $words[$i];
 }
 else 
 {
  $included[] = $words[$i];
 }       
} 

How can I make $included and $ignored a string instead of an array. 

Thanks
Sam Rose

p.s. Also I'm only on the digest, so if you could reply directly to me as
well, that would be a bonus.