[plugin request] remove word starting with xxx from value threadinfo['preview']

[abdobasha2004]

I have a value named threadinfo['preview'], it is in vbulletin script
I want to make a plugin, php code that :
do not allow word starting with http://, so that no url appear in the value threadinfo['preview']

so that for example the value = "http://forums.digitalpoint.com/images/smilies/tongue.gif hi I would like to introduce my self to all, I am new at PHP..."

looks like:"hi I would like to introduce my self to all, I am new at PHP..."

I am really new at PHP and any help would be appreciated