remove part of text in many files

[barka]

i have alot of .php files that contained text like below:

simkop_userlevelsadd.php
simkop_userlevelsdelete.php
simkop_userlevelsedit.php
simkop_userlevelsinfo.php
simkop_userlevelslist.php
simkop_userlevelsview.php
simkop_userlist.php
simkop_userupdate.php

can you please to tell me, how to make reguler expression to remove/delete "simkop_" only, so the result is like below:

userlevelsadd.php
userlevelsdelete.php
userlevelsedit.php
userlevelsinfo.php
userlevelslist.php
userlevelsview.php
userlist.php
userupdate.php

[barka]

apologize, i want to explain what i mean more detail:

i have alot of .php files that contained text like below:

simkop_userlevelsadd.php
simkop_userlevelsdelete.php
simkop_userlevelsedit.php
simkop_userlevelsinfo.php
simkop_userlevelslist.php
simkop_userlevelsview.php
simkop_userlist.php
simkop_userupdate.php

simkop_userlevelsadd
simkop_userlevelsdelete
simkop_userlevelsedit
simkop_userlevelsinfo
simkop_userlevelslist
simkop_userlevelsview
simkop_userlist
simkop_userupdate

can you please to tell me, how to make reguler expression to remove/delete "simkop_" only in text that contained .php, so the result is like below:

userlevelsadd.php
userlevelsdelete.php
userlevelsedit.php
userlevelsinfo.php
userlevelslist.php
userlevelsview.php
userlist.php
userupdate.php

(no change)
simkop_userlevelsadd
simkop_userlevelsdelete
simkop_userlevelsedit
simkop_userlevelsinfo
simkop_userlevelslist
simkop_userlevelsview
simkop_userlist
simkop_userupdate