Just check if its in your array, if so, set it selected:

foreach($array as $value){ ?>
    $selected = in_array($value, $job_tags) ? ' selected ' : '';
    /* Or [selected="selected"] if you dont use html5 yet (which you should) */

    <option value ='<?php echo $value['_id']; ?>' <?php echo $selected; ?>> <?php echo $value['job_name']; ?> </option>
    <?php
}

Your code can be simplified though:

foreach($array as $value){ 
    $selected = in_array($value, $job_tags) ? ' selected="selected" ' : ''; 
    ?>
    <option value="<?=$value['_id']?>" <?=$selected?> > <?=$value['job_name']?> </option>
    <?php
}

I changed the quotes arround the value to doubles, not really a rule, but it is a good practice to do so. The other change is the short echo. Small demo, both do the same:

<php $var = 'foorbar'; ?> <!-- A bit weird, but this is demo-purpose -->

<span><?php echo $var; ?></span>
<span><?=$var?></span>

您可以在字符串 $job_exp_tags 中查找值，如果找到，则设置选定的属性。

foreach($array as $value){
    $selected = strpos($job_exp_tags, $value) ? 'selected' : '';
    echo '<option value="'.$value['_id'].'" '.$selected.'>'.$value['job_name'].'</option>';
}