These are not actual extensions - they are part of the PHP core and cannot be compilled out with compilation options.
Reference V, Array Functions
Reference XII, Class/Object Functions
Reference XXII, Date and Time Functions
Reference XXVIII, Directory Functions
Reference XXIX, DOM Functions
Reference XXXI, Error Handling and Logging Functions
Reference CXVI, Program Execution Functions
Reference XXXV, Filesystem Functions
Reference XLII, Function Handling Functions
Reference XLVII, HTTP Functions
Reference CIX, PHP Options&Information
Reference LXV, Mail Functions
Reference LXVII, Mathematical Functions
Reference LXXVIII, Miscellaneous Functions
Reference LXXXVIII, Network Functions
Reference CI, Output Control Functions
Reference CXXII, Regular Expression Functions (POSIX Extended)
Reference CXLI, String Functions
Reference CXLVI, URL Functions
Reference CXLVII, Variable Handling Functions