read only fields with dependencies, put the following code in: /custom/Extension/modules/<module>/Ext/Dependencies/custom_readonly_fields_logic.php

custom_readonly_fields_logic.php

<?php
// Documentation.- http://support.sugarcrm.com/Documentation/Sugar_Developer/Sugar_Developer_Guide_7.7/Architecture/Sugar_Logic/Dependency_Actions/
$dependencies['Meetings']['readonly_fields'] = array(
        'hooks' => array("edit"), // values : "edit", "view", "save" and "all" 
        'trigger' => 'equal($status,"Held")', //Optional, the trigger for the dependency. Defaults to 'true'.
        'triggerFields' => array('status'), // The list of fields to watch for change events. When changed, the trigger expressions will be recalculated.
        'onload' => true, // Whether or not to trigger the dependencies when the page is loaded.
        //Actions is a list of actions to fire when the trigger is true
        // You could list multiple fields here each in their own array under 'actions'
        'actions' => array(
            array(
                'name' => 'ReadOnly', // actions: ReadOnly, SetOptions, SetPanelVisibility, SetRequired, SetValue, SetVisibility
                //The parameters passed in will depend on the action type set in 'name'
                'params' => array(
                    'target' => 'status',
                    'value' => 'true',
                ),
            ),
        ),
        // The list of dependencies to execute when the trigger expression is false
        'notActions' => array(
        ),
    );

Actions:
ReadOnly.- Is used to determine if a field is editable or not based on a formula.
SetOptions.- Is used to set the options list of a dropdown field based on a formula.
SetPanelVisibility.- Is used to determine the visibility of a record view panel based on a formula.
SetRequiredThe.- Is used to determine if a field is required.
SetValue.- Is used to set the value of a field based on a formula.
SetVisibility.- Is used to determine the visibility logic of a field based on a formula.