/**
    * function to create a html select box and set the selected value if the item is found in post
    *
    * @param string $elementId and id of html element
    * @param array $options of items to add to select box value => display text
    * @param string $attributes of attributes to add e.g class=''
    * @return string
    */
    function createSelectHtml($elementId, $options, $attributes=''){
        $returnHtml="";
        //start building return html
        $returnHtml.="\n<select name='{$elementId}' id='{$elementId}'>\n";
        //default selected item to blank (if not found defaults to 1st item)
        $selectedValue='';
        //check if a post has been sent containing this item
        if ($_POST && isset($_POST[$elementId])){
            //a post has been done set selected to post value
            $selectedValue=$_POST[$elementId];
        }
        //loop through options adding to html
        foreach ($options as $id => $value){
            //check if the item we are adding is the selected item
            if ($id==$selectedValue){
                $isSelected=" selected='selected'";
            }else{
                $isSelected="";
            }
            //add this option to the return Html
            $returnHtml.= " <option value='{$id}'{$isSelected}>{$value}</option>\n";
        }
        //end select box
        $returnHtml.= "</select>\n";
        //if the item has been submitted check if it is valid
        if ($_POST){
            //display error message
            if (!selectIsValid($elementId, $options)){
                $returnHtml.= "<span class='error'>Please select a valid option from the list provided.</span>\n";
            }
        }
        //return html
        return $returnHtml;
    }
    /**
    * function to check if a valid option has been posted to a form
    *
    * @param string $elementId and id of html element
    * @param array $options of items to add to select box value => display text
    * @return boolean
    */
    function selectIsValid($elementId, $options){
        $isValid=false;
        //loop through options adding to html
        foreach ($options as $id => $value){
            if ($id!='' && $id==$_POST[$elementId]){
                $isValid=true;
            }
        }
        return $isValid;
    }

<form method="post">
<?php
    $validOptions=array('' => 'Please select', 1 => 'Matt', 2 => 'John', 3 => 'Smith');
    echo "<label>Please select:</label>" . createSelectHtml('aSelectBox', $validOptions) . "";
    echo "<p>&nbsp;</p>";
    echo "<label>Please select:</label>" . createSelectHtml('aSelectBox1', $validOptions) . "";
    echo "<p>&nbsp;</p>";
    if ($_POST){
        if(selectIsValid('aSelectBox', $validOptions)){
            echo "<p>The form is valid</p>";
        }
    }
?>
<input type="submit">
</form>