Here's my list so far.

ASP Functions PHP Functions
==================== ===============
Left(str,length) substr(str,start,length)
Right(str,length) substr(str,start,length)
Mid(str,start,stop) substr(str,start,length)
Len(str) strlen(str)
Trim(str) trim(str)
LTrim(str) ltrim(str)
RTrim(str) rtrim(str)
UCase(str) strtoupper(str)
LCase(str) strtolower(str)
InStr(str,value) ---DON'T KNOW---