where can i declare my custom functions in magento

I want to declare some php functions and i would like to call those functions in various places in magento.Usually in my core php projects i'm declaring php functions in functions.php and i include that file in all 开发者_运维百科pages.I'm not familiar with MVC structure.So where can i declare these kind of functions.

Thanks

Edit :-

Mango_Myfunc.xml (app/etc/modules)

<?xml version="1.0"?>
<config>
    <modules>
        <Mango_Myfunc>
            <active>true</active>
            <codePool>local</codePool>
        </Mango_Myfunc>
    </modules>
</config>

Config.xml(app/code/local/Mango/Myfunc/etc/configure.xml)

<?xml version="1.0"?>
<config>
    <modules>
        <Mango_Myfunc>
            <version>0.1.0</version>
        </Mango_Myfunc>
    </modules>
    <global>
        <helpers>
            <Myfunc>
                <class>Mango_Myfunc_Helper</class>
            </Myfunc>
        </helpers>
    </global>
</config>

Data.php(app/code/local/Mango/Myfunc/helper/Data.php)

class Mango_Myfunc_Helper_Data extends Mage_Core_Helper_Abstract
{

public function short_str ($str, $len, $suf = '...') {
    if (strlen($str) > $len)
        return substr($str, 0, $len - strlen($suf) ) . $suf;

    return $str;
}

}

This is what i added

i used bellow one to call the function in list.phtml

echo $this->helper('Myfunc/Data')->short_str("test","3"); got the error

Fatal error: Class 'Mage_Myfunc_Helper_Data' not found

Magento has helper classes for those kind of methods. So make your extensions and add your methods and you can then later call them like follows

Mage::helper('yourextension/yourhelper')->yourMethod(); 

Or you can make a library class out of your common methods.