mysql - in_array() expects parameter 2 to be array, null given error -

i have form displays checkbox each engineer in table. when form submitted if engineer's checkbox isn't ticked removed job ($diary_id).

the code @ moment works page briefly displays error before going next page.

in_array() expects parameter 2 array, null given error

the code on form...

  $sql = "select * users type = 'engineer' order first_name";   $result = mysql_query($sql) or die(mysql_error());   while ($row = mysql_fetch_array($result)){ ?>        <div style="width:70px; float:left">       <input type="hidden" name="engineer_on_page[]" value="<? echo $row['id'];  ?>"/>           <input type="checkbox" name="engineer[]" <?           $eng_id= $row['id'];                $result2 = mysql_query("select * calls_engineers call_ce = '$diary_id' , engineer_ce = '$eng_id'");               if((mysql_num_rows($result2)) > 0)               { echo 'checked="checked"';              ?> value="<? echo $row['id']; ?>" />           <? echo '   '.$row['first_name']; }              else {                  echo "value=".$row['id']." />";           echo '   '.$row['first_name'];              }                ?>       </div>    <? } ?> 

the mysql remove unselected engineers

foreach($_post['engineer_on_page'] $engineer_id) {             $sql = "select * calls_engineers call_ce = '$diary_id' , engineer_ce = '$engineer_id'";             $result = mysql_query($sql)or die(mysql_error());                 if(!in_array($engineer_id, $_post['engineer'])){                     if(mysql_num_rows($result) > 0){                         $sql = "delete calls_engineers engineer_ce = '$engineer_id' , call_ce = '$diary_id'";                         $result = mysql_query($sql)or die(mysql_error());                     }                            }              } 

i have tried initialise $_post['engineer'] array ($_post['engineer'] = array();) before foreach loop stops error, stops page working, no error messages, doesn't work more.

i aware page needs updated mysqli resolve issue before do.

thanks

if 1 engineer checked, value might string, not array.

then, add is_array() before calling in_array():

if(is_array($_post['engineer']) && !in_array($engineer_id, $_post['engineer'])){     // code           } 

there lot of ways structure that, should idea.