PHP中用hash实现的数组_php技巧

static inline ulong zend_inline_hash_func(const char *arKey, uint nKeyLength) 
{ 
register ulong hash = 5381;                                                   //此处初始值的设置有什么玄机么？ 
/* variant with the hash unrolled eight times */ 
for (; nKeyLength >= 8; nKeyLength -= 8) {                         //这种step=8的方式是为何？ 
hash = ((hash << 5) + hash) + *arKey++; 
hash = ((hash << 5) + hash) + *arKey++; 
hash = ((hash << 5) + hash) + *arKey++; 
hash = ((hash << 5) + hash) + *arKey++;                         //比直接*33要快 
hash = ((hash << 5) + hash) + *arKey++; 
hash = ((hash << 5) + hash) + *arKey++; 
hash = ((hash << 5) + hash) + *arKey++; 
hash = ((hash << 5) + hash) + *arKey++; 
} 
switch (nKeyLength) { 
case 7: hash = ((hash << 5) + hash) + *arKey++; /* fallthrough... */                             //此处是将剩余的字符hash 
case 6: hash = ((hash << 5) + hash) + *arKey++; /* fallthrough... */ 
case 5: hash = ((hash << 5) + hash) + *arKey++; /* fallthrough... */ 
case 4: hash = ((hash << 5) + hash) + *arKey++; /* fallthrough... */ 
case 3: hash = ((hash << 5) + hash) + *arKey++; /* fallthrough... */ 
case 2: hash = ((hash << 5) + hash) + *arKey++; /* fallthrough... */                     
case 1: hash = ((hash << 5) + hash) + *arKey++; break; 
case 0: break; 
EMPTY_SWITCH_DEFAULT_CASE() 
} 
return hash;//返回hash值 
} 
 
ps：对于以下函数，仍有两点不明： 
hash = 5381设置的理由？ 
这种step=8的循环方式是为了效率么？