尝试检查键是否存在..

Try to check if the keys does exist ..

$b 和 $subkey 未在您提供的函数中定义.

$b and $subkey not defined inside the function you provided.

function yiw_subval_sort( $a, $subkey ) {
    if( is_array( $a ) AND ! empty( $a ) ) {
        foreach( $a as $k => $v ) {
            if( isset( $b[$k] ) && isset( $v[$subkey] ) )
                $b[$k] = strtolower( $v[$subkey] );
    }