Parse error PHP unexpected " -

I'm getting the following error in my code. (T_VARIABLE) or number (T_VARIABLE) or number (T_NUM_STRING)

Pars error: syntax error, unexpected "(T_ENCAPSED_AND_WHITESPACE), identifier (T_STRING) P> What do I need to change in the following line to work? Thank you

  if (dbQuery ("users update sessionid = '$ sessionid' WHERE id = $ _ Session ['id'] ")) {back true;}    
 
 
  If (dbQuery ("update user sessionid = You can simply simplify your code:  
 '$ sessionid'  Return dbQuery ("Update users sessionid = '$ session' WHERE id =". $ _SESSION ['id']);