convenience function for ?: short ternary assignment syntax; avoids need for isset() checks.

is.php

<?php

/**
 * performs validation on a variable, returning the variable's value on success.
 *
 * @author    Adrian <adrian@enspi.red>
 * @copyright 2013
 * @since     2014-08-21  allows a validator callback
 * @license   MIT <http://opensource.org/licenses/MIT>
 *
 * @example<code>
 *  <?php
 *  // originally developed to replace "isset" checks in ternary operators
 *    $declared = "hello, world!";
 *    $default  = "goodbye, world.";
 *
 *    // allows you to do:
 *    print is( $declared   )?: $default; // prints "hello, world!"
 *    print is( $undeclared )?: $default; // prints "goodbye, world."
 *
 *    // instead of:
 *    print isset( $declared   )? $declared:   $default; // prints "hello, world!"
 *    print isset( $undeclared )? $undeclared: $default; // prints "goodbye, world."
 *
 *
 *  // also allows for validation using a callback:
 *    $object = (object)['email':'email@example.com"];
 *    $string = "foo";
 *
 *    // named functions
 *    print is( $string,"is_string" )?: "not a string"; // prints "foo"
 *    print is( $object,"is_string" )?: "not a string"; // prints "not a string"
 *
 *    // closures
 *    $validator = function( $v ){
 *      return is_object( $v )
 *        && isset( $v->email )
 *        && filter_var( $v->email,FILTER_VALIDATE_EMAIL );
 *    };
 *    print is( $object,$validator )? "thank you": "you forgot your email address"; // prints "thank you"
 *    
 * </code>
 * 
 * @param mixed    $var         the variable to check (need not be declared in advance)
 * @param callable $validator   validation method (in the form bool validator( $var ))
 * @return mixed                the value of $var on success; null otherwise
 */
function is( &$var,callable $validator=null ){
    if( $validator && ! $validator( $var )){ return null; }
    return $var;
}