The goal of the new memory manager (PHP 5.2 and later) is reducing memory allocation overhead and speedup memory management.
The new manager's "configure" has no "--disable-zend-memory-manager" option, but it has "--enable-malloc-mm" instead. It is enabled by default in DEBUG build and disabled by default in RELEASE built. It allows select malloc/emalloc at runtime. So you can use internal and external memory debuggers without recompilation.
The patch allows tweaking memory manager with ZEND_MM_MEM_TYPE and ZEND_MM_SEG_SIZE environment variables. Default values are "malloc" and "256K". Dependent on target system you can also use "mmap_anon", "mmap_zero"