致命错误：for循环中PHP数组上的字符串错误不支持[]运算符

I have this PHP Code:

for($i=1; $i<=$_POST["counter"]; $i++)
{
    if($_POST["checkbox$i"])
    {
        //select the billing_pdf row
        $sql="SELECT * from billing_pdf_archive where sequence = '".$_POST["checkbox$i"]."' ";
        $rs=mysql_query($sql,$conn);
        while($result=mysql_fetch_array($rs))
        {
            //now select all contacts for the company who receive all accounts emails
            $sql2="SELECT * from contacts where company_sequence = '".$result["customer_sequence"]."' and receive_accountsemails = 'yes' ";
            $rs2=mysql_query($sql2,$conn) or die(mysql_error());
            if(mysql_num_rows($rs2) > 0)
            {
                while($contacts2=mysql_fetch_array($rs2))
                {
                    //generate the list of emails address in an array
                    $emails_list[] = $contacts2["email"];
                }
                $emails_list = implode(',',$emails_list);
            }
        }

        $ins_sql2="INSERT into email_attachments (email_seq, attachment) values ('".$email_sequence."', '".$result["pdf"]."') ";
        $ins_rs2=mysql_query($ins_sql2,$conn) or die(mysql_error());

        $up_sql="UPDATE emails set emailto = '".$emails_list."' where sequence = '".$email_sequence."' ";
        $up_rs=mysql_query($up_sql,$conn);
    }
}

but I am getting this error:

Fatal error: [] operator not supported for strings in

on the line that says:

$emails_list[] = $contacts2["email"];

I am using the same array code on other pages (without the for loop) and they work fine

what am I doing wrong?

you fill the array $emails_list[] in the second while with $contacts2["email"]. in the next line, you implode the array to a string.

for the next result (first while loop) $emails_list is a string. now you can't convert a string to an array

try this:

        $emailsListData = array();
        while($contacts2=mysql_fetch_array($rs2))
        {
            //generate the list of emails address in an array
            $emailsListData[] = $contacts2["email"];
        }
        $emails_list = implode(',',$emailsListData);

You have already used $emails_list as a string some where in your page like $emails_list = 7;

and again using it like

$emails_list[] = array(1,2,4);