replacing characters

[benjy]

say i have a simple form and a text file. and the form edits the text file using php. in the form the user will type html. what i want to know is, that if a user types in a particular word how can php replace that word with a string of html.

for example. if a user types in the word "cat" into the form and submits it how do i get php to then recognise that the user has typed in "cat" and replace it with lets say <a href="http://www.somewere.com">go here"</a>

[hkucsis]

Taken from
this thread

$str = preg_replace("/\b$keyword\b/i", "$replacement", $str);

// in your case
$keyword = "cat" ;
$replacement = '<a href="http://www.somewere.com">go here"</a>' ;