PHP的in_array函数如何工作？

Question

I am not a computer scientist or have strong algorithmic knowledge, but today I began to wonder how does PHP's in_array method work under the hood. It seems pretty straight-forward, loop through all the array's elements and check the current element against a given value. But even with larger arrays it still works very quickly. How?

<?php
$distros = ["Mint", "Debian", "Ubuntu", "Fedora", "CentOS", "Arch"];
if (in_array("Ubuntu", $distros)) {
    echo "Got Ubuntu";
}

Answer 1

Here it is:

/* void php_search_array(INTERNAL_FUNCTION_PARAMETERS, int behavior)
 * 0 = return boolean
 * 1 = return key
 */
static inline void php_search_array(INTERNAL_FUNCTION_PARAMETERS, int behavior) /* {{{ */
{
    zval *value,                /* value to check for */
         *array,                /* array to check in */
         *entry;                /* pointer to array entry */
    zend_ulong num_idx;
    zend_string *str_idx;
    zend_bool strict = 0;       /* strict comparison or not */

#ifndef FAST_ZPP
    if (zend_parse_parameters(ZEND_NUM_ARGS(), "za|b", &value, &array, &strict) == FAILURE) {
        return;
    }
#else
    ZEND_PARSE_PARAMETERS_START(2, 3)
        Z_PARAM_ZVAL(value)
        Z_PARAM_ARRAY(array)
        Z_PARAM_OPTIONAL
        Z_PARAM_BOOL(strict)
    ZEND_PARSE_PARAMETERS_END();
#endif

    if (strict) {
        ZEND_HASH_FOREACH_KEY_VAL(Z_ARRVAL_P(array), num_idx, str_idx, entry) {
            ZVAL_DEREF(entry);
            if (fast_is_identical_function(value, entry)) {
                if (behavior == 0) {
                    RETURN_TRUE;
                } else {
                    if (str_idx) {
                        RETVAL_STR_COPY(str_idx);
                    } else {
                        RETVAL_LONG(num_idx);
                    }
                    return;
                }
            }
        } ZEND_HASH_FOREACH_END();
    } else {
        ZVAL_DEREF(value);
        if (Z_TYPE_P(value) == IS_LONG) {
            ZEND_HASH_FOREACH_KEY_VAL(Z_ARRVAL_P(array), num_idx, str_idx, entry) {
                if (fast_equal_check_long(value, entry)) {
                    if (behavior == 0) {
                        RETURN_TRUE;
                    } else {
                        if (str_idx) {
                            RETVAL_STR_COPY(str_idx);
                        } else {
                            RETVAL_LONG(num_idx);
                        }
                        return;
                    }
                }
            } ZEND_HASH_FOREACH_END();
        } else if (Z_TYPE_P(value) == IS_STRING) {
            ZEND_HASH_FOREACH_KEY_VAL(Z_ARRVAL_P(array), num_idx, str_idx, entry) {
                if (fast_equal_check_string(value, entry)) {
                    if (behavior == 0) {
                        RETURN_TRUE;
                    } else {
                        if (str_idx) {
                            RETVAL_STR_COPY(str_idx);
                        } else {
                            RETVAL_LONG(num_idx);
                        }
                        return;
                    }
                }
            } ZEND_HASH_FOREACH_END();
        } else {
            ZEND_HASH_FOREACH_KEY_VAL(Z_ARRVAL_P(array), num_idx, str_idx, entry) {
                if (fast_equal_check_function(value, entry)) {
                    if (behavior == 0) {
                        RETURN_TRUE;
                    } else {
                        if (str_idx) {
                            RETVAL_STR_COPY(str_idx);
                        } else {
                            RETVAL_LONG(num_idx);
                        }
                        return;
                    }
                }
            } ZEND_HASH_FOREACH_END();
        }
    }

    RETURN_FALSE;
}
/* }}} */

/* {{{ proto bool in_array(mixed needle, array haystack [, bool strict])
   Checks if the given value exists in the array */
PHP_FUNCTION(in_array)
{
    php_search_array(INTERNAL_FUNCTION_PARAM_PASSTHRU, 0);
}