Check if variable is_undefined in PHP

问题:

In PHP, I want to check if a variable has not been set/defined, where setting a variable NULL is considered set/defined.

I'm aware everything here: http://php.net/manual/en/types.comparisons.php including isset(), empty(), and is_null(). None of these appear to be what I'm looking for. Consider the following example:

<?php 
$myNull = null;
echo 'isset($myNull): "'.isset($myNull).'"<br />';
echo '$myNull value = "'.$myNull . '"<br />';

echo "<br />";

echo 'isset($myUndefined): "'.isset($myUndefined).'"<br />';
echo '$myUndefined value = "'.$myUndefined . '"<br />';
?>

This example outputs something like:
isset($myNull): ""
$myNull value = ""

isset($myUndefined): ""
Notice: Undefined variable: myUndefined in C:\wamp\www\plm\temp4.php on line 9
$myUndefined value = ""

I want to know if a variable is Undefined as it says above in the notice. I want a function, call it "is_undefined", where

$myNull = null;
is_undefined($myNull); // is false
is_undefined($myUndefined); // is true

Anyone? Thanks in advance.

回答1:

I haven´t used it yet - but I think that "get_defined_vars" should be worth a look... http://php.net/manual/en/function.get-defined-vars.php

I would give it a try and dump the result.

回答2:

I think that get_defined_vars is a good candidate for such job:

array_key_exists('myNull', get_defined_vars());

Should do what you expect.

If you work on a global context, you can also use:

array_key_exists('myNull', $GLOBALS);

回答3:

If you want an is_undefined function I would prefer not to work with arrays so I would do this:

function is_undefined(&$test) {
    return isset($test) && !is_null($test);
}

So when you echo isset($myNull); it converts the boolean(true) to "". thats why the value is blank. If you want to see it on the screen you can do var_dump(isset($myNull)); that will display if it's true or false.

Also you have an echo of $myUndefined but it's not set yet so that's why you get a warning. What you want to do is:

if (!empty($myUndefined)) {
    // variable is defined so do something with it
    echo '$myUndefined value = "' . $myUndefined . '"<br />';
} else {
    echo 'Oops, $myUndefined is Undefined!<br />";
}

Here is a brief overview of isset() vs. is_null() vs. empty()

$foo = null;
// isset($foo) == true;
// empty($foo) == true;
// is_null($foo) == true;

// Notice I don't set $foo2 to anything
// isset($foo2) == false;
// empty($foo2) == true;
// is_null($foo2) throws a notice!

$foo3 = false;
// isset($foo2) == true;
// empty($foo2) == true;
// is_null($foo2) == false;

$foo4 = 1234;
// isset($foo2) == true;
// empty($foo2) == false;
// is_null($foo2) == false;

回答4:

You can use compact() for this too, if the variable you give it isn't in the symbol table it returns an empty array, otherwise an array containing the variable name/value pair, just cast the result to a boolean

<?php

$myNull = null;

$isDefined = (bool) compact('myNull'); // true

$otherIsDefined = (bool) compact('myUndefined'); // false 

回答5:

If you are using OOP then use overloading __isset() this function will execute when you are trying to access a variable that is not defined anywhere. example:

 public function __isset($name) {
    echo 'Hello';
 }

Thus, will avoid any error message or notice related to undefined variable

回答6:

Yes, just like Mr. Jonathan mentioned above, we could use array_key_exists() + $GLOBALS rather than get_defined_vars() to identify Undefined variable to null

$x;  // $x is undefined 
$y=null;  // $y is defined and is NULL type variable with the only null value
$z=[];    // $z is an array object

if( array_key_exists('x', $GLOBALS) && is_null($x) ) echo "\$x exists and is null\n";
if( array_key_exists('y', $GLOBALS) && is_null($y) ) echo "\$y exists and is null\n";
if( array_key_exists('z', $GLOBALS) && is_null($z) ) echo "\$he exists and is null\n";

// output 
$y exists and is null