Most frequently words in a text

Hello everybody, I am creating an algorithm to identify the most frequently words on different sentences, using the following code: // store in $arr_frequencyids all different words for($i=0; $i<count($arr_phrase); $arr_phrase) { array_push($arr_frequencyids,explode(" ",$arr_phrase[$i])); } // use a hash using words as keys, and increment as frequency for ($i=0; $i<count($arr_frequencywords); $i++) { $str_thisword = $arr_frequencywords[$i]; $hash_frequencywords[$str_thisword]++; } // descendent sort based on values arsort($hash_frequencywords); // store in $arr_20mostusedwords the 20 most frequent words $arr_20mostusedwords = array_slice(array_keys($hash_frequencywords,0,20)); But... is not working. It is not counting the ocurrences. It stores as a long string, even I used explode or trying a direct array addition (arr1 += arr2). Warning: Wrong parameter count for array_keys() in /home/mipyme/public_html/jpanel/index.php on line 204 Warning: Wrong parameter count for array_slice() in /home/mipyme/public_html/jpanel/index.php on line 204 Warning: array_slice() [function.array-slice]: The first argument should be an array in /home/mipyme/public_html/jpanel/index.php on line 205 Could you give me some direction? (the information comes from a DB, but for simplifying process I asume an array instead a while DB fetch row) Thank you, Alejandro