$loc_considered1 = implode(', ',$loc_considered); is the problem line. implode() expects parameter one to be a string (which it is) and parameter two to be an array, which it probably isn't. Where is the form that produces the $_POST['loc_considered'] variable?
Can you post that code?
Your code was hit by a train... on line 1, you open a php code block and you do not close it before inserting standard HTML:
<td>Location Considered </td>
<td align="left" colspan="2">
Then you open another PHP code block on line 6 without closing the first unneccessary code block (?>).
I could rewrite the code above to acheive your goal, but what would you learn?
Have a look here for an explanation of how to access the variables in the multiple select box (line 12).
Tidy your code, debug the value of $_POST when the action page loads (print_r($_POST); die();), and post your updated code if you still have problems.