为类定义常量 
这个内容比较简单，只涉及到一组函数，可以查看zend/zend_api.h 
zend_api int zend_declare_class_constant(zend_class_entry *ce, const char *name, size_t name_length, zval *value tsrmls_dc); 
zend_api int zend_declare_class_constant_null(zend_class_entry *ce, const char *name, size_t name_length tsrmls_dc); 
zend_api int zend_declare_class_constant_long(zend_class_entry *ce, const char *name, size_t name_length, long value tsrmls_dc); 
zend_api int zend_declare_class_constant_bool(zend_class_entry *ce, const char *name, size_t name_length, zend_bool value tsrmls_dc); 
zend_api int zend_declare_class_constant_double(zend_class_entry *ce, const char *name, size_t name_length, double value tsrmls_dc); 
zend_api int zend_declare_class_constant_stringl(zend_class_entry *ce, const char *name, size_t name_length, const char *value, size_t value_length tsrmls_dc); 
zend_api int zend_declare_class_constant_string(zend_class_entry *ce, const char *name, size_t name_length, const char *value tsrmls_dc); 
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