The constants below are always available as part of the PHP core.
PHP_OUTPUT_HANDLER_START
     (integer)
   Indicates that output buffering has begun.
PHP_OUTPUT_HANDLER_WRITE
     (integer)
   Indicates that the output buffer is being flushed, and had data to output.
Available since PHP 5.4.
PHP_OUTPUT_HANDLER_FLUSH
     (integer)
   Indicates that the buffer has been flushed.
Available since PHP 5.4.
PHP_OUTPUT_HANDLER_CLEAN
     (integer)
   Indicates that the output buffer has been cleaned.
Available since PHP 5.4.
PHP_OUTPUT_HANDLER_FINAL
     (integer)
   Indicates that this is the final output buffering operation.
Available since PHP 5.4.
PHP_OUTPUT_HANDLER_CONT
     (integer)
   Indicates that the buffer has been flushed, but output buffering will continue.
     As of PHP 5.4, this is an alias for
     PHP_OUTPUT_HANDLER_WRITE.
    
PHP_OUTPUT_HANDLER_END
     (integer)
   Indicates that output buffering has ended.
     As of PHP 5.4, this is an alias for
     PHP_OUTPUT_HANDLER_FINAL.
    
PHP_OUTPUT_HANDLER_CLEANABLE
     (integer)
   Controls whether an output buffer created by ob_start() can be cleaned.
Available since PHP 5.4.
PHP_OUTPUT_HANDLER_FLUSHABLE
     (integer)
   Controls whether an output buffer created by ob_start() can be flushed.
Available since PHP 5.4.
PHP_OUTPUT_HANDLER_REMOVABLE
     (integer)
   Controls whether an output buffer created by ob_start() can be removed before the end of the script.
Available since PHP 5.4.
PHP_OUTPUT_HANDLER_STDFLAGS
     (integer)
   
     The default set of output buffer flags; currently equivalent to
     PHP_OUTPUT_HANDLER_CLEANABLE |
     PHP_OUTPUT_HANDLER_FLUSHABLE |
     PHP_OUTPUT_HANDLER_REMOVABLE.
    
Available since PHP 5.4.