CASESAFEID() apex equivalent

Per the Apex docs on primitive datatypes, “Note that if you set [an] ID to a 15-character value, Apex automatically converts the value to its 18-character representation.” So it should be enough to assign your 15 character id to an ID-typed variable, and then back to a string, like so:

string idStr = '001E000000nwg7g';   // 15 character id
id idval = idStr;                   // assign to ID variable
idStr = idval;                      // back to string to prove the point
system.debug('18 char: ' + idStr);  // note that you could just append idval instead
                                    // of converting to string first

Remember that assigning an invalid id string to an ID-typed variable will throw an exception; if there’s any chance that your 15 character value may not be a valid id, wrap the assignment in a try/catch block.