The operations allowed on buffers can be controlled
by passing one of the
buffer control flags
to ob_start()'s third
flags parameter.
If omitted, all operations are allowed by default.
If 0 is used instead,
the buffer cannot be flushed, cleaned or removed
but it's contents can still be retrieved.
PHP_OUTPUT_HANDLER_CLEANABLE allows
ob_clean() to clean the contents of the buffer.
The absence of the PHP_OUTPUT_HANDLER_CLEANABLE flag
will not prevent ob_end_clean()
or ob_get_clean() from clearing the contents of the buffer.
PHP_OUTPUT_HANDLER_FLUSHABLE allows
ob_flush() to flush the contents of the buffer.
The absence of the PHP_OUTPUT_HANDLER_FLUSHABLE flag
will not prevent ob_end_flush()
or ob_get_flush() from flushing the contents of the buffer.
PHP_OUTPUT_HANDLER_REMOVABLE allows
ob_end_clean(), ob_end_flush(),
ob_get_clean() or ob_get_flush()
to turn off the buffer.
PHP_OUTPUT_HANDLER_STDFLAGS,
the combination of the three flags will allow each of the three operations
to be performed on the buffer.