Zend查询大于或等于

I have my query which works perfectly in MySQL

SELECT * FROM accounts WHERE location_id=1 AND created_at>=*timestamp*

I have a model for the Accounts table in Zend. I used

$accounts->where(array('location_id' => 1))

and when I try to use another where clause

->where("created_at >= $timestamp")

Zend throws me this error:

"SQLSTATE[42000]: Syntax error or access violation: 1064 You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near ')' at line 7"

I have also tried to hardcode my $timestamp variable, delete the first where and use just the second one, and even tryed to change the syntax to

->where("created_at >= " . $timestamp)

None of them worked. Any ideeas?

LATER EDIT

I figured it out, it seems it was a syntax problem. This worked for me:

where("created_at >= '$timestamp'");

->where("start >= ?", $timestamp)

(in ZF1). Also, in the query you said works perfectly the column is named created_at.