Where can I find the function "require" in the PHP source?

I have checked the PHP source code and can not find the "require" function. I only found a piece code relatives to it on main/main.c from line 976

switch (EG(current_execute_data)->opline->extended_value) {
    case ZEND_EVAL:
        function = "eval";
        is_function = 1;
        break;
    case ZEND_INCLUDE:
        function = "include";
        is_function = 1;
        break;
    case ZEND_INCLUDE_ONCE:
        function = "include_once";
        is_function = 1;
        break;
    case ZEND_REQUIRE:
        function = "require";
        is_function = 1;
        break;
    case ZEND_REQUIRE_ONCE:
        function = "require_once";
        is_function = 1;
        break;
    default:
        function = "Unknown";
}

Do you know which file defines the "function"?