Static xmlDefaultSAXHandler 

#[unsafe(no_mangle)]
pub static mut xmlDefaultSAXHandler: _xmlSAXHandlerV1

const xmlSAXHandlerV1 xmlDefaultSAXHandler (globals.c 2.15.3).

#[no_mangle] data symbol that C consumers (PHP’s DOM/XML extensions) MUTATE (they overwrite the error/warning callbacks on the shared default handler). Rust places non-mut statics in read-only memory; a write to it would segfault. Make it static mut so it lives in writable data.