官方手册（http://us.php.net/manual/en/function.natsort.php）
bool natsort ( array &$array )
This function implements a sort algorithm that orders alphanumeric strings in the way a human being would while maintaining key/value associations. This is described as a "natural ordering". An example of the difference between this algorithm and the regular computer string sorting algorithms (used in sort()) can be seen in the example below.
据官方手册还可以得到这样的结果：
img1.png img2.png img10.png img12.png
显然这很适合对类似文件名的排序。从结果看这种自然算法应该是去掉头和尾的非数字部分，然后对留下来的数字部分进行排序，究竟是不是，还是看一下php源码吧。
//从ext/standard/array.c抽取的相关代码如下
static int php_array_natural_general_compare(const void *a, const void *b, int fold_case) /* {{{ */
{
Bucket *f, *s;
zval *fval, *sval;
zval first, second;
int result;
f = *((Bucket **) a);
s = *((Bucket **) b);
fval = *((zval **) f->pData);
sval = *((zval **) s->pData);
first = *fval;
second = *sval;
if (Z_TYPE_P(fval) != IS_STRING) {
zval_copy_ctor(&first);
convert_to_string(&first);
}
if (Z_TYPE_P(sval) != IS_STRING) {
zval_copy_ctor(&second);
convert_to_string(&second);
}
result = strnatcmp_ex(Z_STRVAL(first), Z_STRLEN(first), Z_STRVAL(second), Z_STRLEN(second), fold_case);
if (Z_TYPE_P(fval) != IS_STRING) {
zval_dtor(&first);
}
if (Z_TYPE_P(sval) != IS_STRING) {
zval_dtor(&second);
}
return result;
}
/* }}} */
static int php_array_natural_compare(const void *a, const void *b TSRMLS_DC) /* {{{ */
{
return php_array_natural_general_compare(a, b, 0);
}
/* }}} */
static void php_natsort(INTERNAL_FUNCTION_PARAMETERS, int fold_case) /* {{{ */
{
zval *array;
if (zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "a", &array) == FAILURE) {
return;
}
if (fold_case) {
if (zend_hash_sort(Z_ARRVAL_P(array), zend_qsort, php_array_natural_case_compare, 0 TSRMLS_CC) == FAILURE) {
return;
}
} else {
if (zend_hash_sort(Z_ARRVAL_P(array), zend_qsort, php_array_natural_compare, 0 TSRMLS_CC) == FAILURE) {
return;
}
}
RETURN_TRUE;
}
/* }}} */
/* {{{ proto void natsort(array &array_arg)
Sort an array using natural sort */
PHP_FUNCTION(natsort)
{
php_natsort(INTERNAL_FUNCTION_PARAM_PASSTHRU, 0);
}
/* }}} */
虽然是第一次查看php的内核代码，不过凭借多年看代码的经验，还是很容易找到这个自然排序算法的核心就是函数：strnatcmp_ex（位于ext/standard/strnatcmp.c文件中）。
