How do I change words in post texts?

[Emeralda]

$censoredword = array(
'word1',
'word2'
);

$changedword = array(
'word3',
'word4'
);

$this->post['message'] = str_ireplace($censoredword, $changedword, $this->post['message']);

And how do the replacement work exactly? Is it something like

PHP Code:

$censoredword = array(
'swear word1',
'swear word2'
);

$changedword = array(
'swear word1 change',
'swear word2 change'
);

$this->post['message'] = str_ireplace($censoredword, $changedword, $this->post['message']); 


Or

PHP Code:

$censoredword = array(
'swear word1',
'swear word2'
);

$changedword = array(
'new word for all swears'
);

$this->post['message'] = str_ireplace($censoredword, $changedword, $this->post['message']);