Quick Tip: How To Check if a Variable Is Set in PHP

This quick PHP tip guide will explain how to check if a variable is set and if it is set, what its value is - this is a very common task in programming.

When checking whether a variable is set in PHP, our first reaction may be to use the isset() function. While this works in most cases, if we look at the definition of the isset() function, we will see the following:

isset(): Determines whether a variable has been declared and is not NULL.

However, in PHP we can set the variable to NULL:

<?php
    $variable = NULL;
    var_dump(isset($variable));

This assignment will return false - even if we have explicitly set the variable to NULL! This is a situation that may cause some confusion in PHP. In the following we will look at the isset() method and when to use it, and what other methods we can use when isset() does not work.

empty() and is_null() appear

While the isset() method can help us determine if a variable has some value set, it will not help us if the variable is set to NULL. To handle this situation, we need to use the empty() or is_null() functions. The empty() function will determine whether the variable is empty. It will be valid for the following situations (i.e. return true):

• "" (empty string)
• 0 (integer 0)
• 0.0 (floating point number 0)
• "0" (String 0)
• NULL
• FALSE
• array() (empty array)
• $var; (variable declared but without value)

This means we can use empty() to determine whether the variable is empty. In this case, both NULL and false are considered empty:

<?php
    $variable = NULL;
    var_dump(empty($variable));

We can also use the is_null() function. This function determines whether the variable is null.

The following is an example of how to use is_null() to determine if a variable is null:

<?php
    $variable = NULL;
    var_dump(is_null($variable));

The most important thing when dealing with PHP variables is to understand the difference between isset(), is_null() and empty(). All three functions can be used to determine the state of variables, but they behave differently. The following table can be used to highlight the differences between these functions.

"""foo"NULLFALSE0 Undefined empty()TrueFalseTrueTrueTrueTrueTrueTrueis_null() True FalseFalseFalseTrueFalseisset()False True (Error) TrueTrueTrueFalse

True<🎜><🎜>True<🎜><🎜>True<🎜><🎜> False<🎜><🎜> <🎜><🎜>

Is there a single function that we can use?

Using get_defined_vars() will return an associative array with the key as the variable name and the value as the variable value. We still can't use isset(get_defined_vars()['variable']) here because the key may exist and the value is still null, so we have to use array_key_exists('variable', get_defined_vars()).

Example:

<?php
    $variable = NULL;
    var_dump(isset($variable));

Using get_defined_vars() this way, we can 100% determine whether we are checking whether the variable is set.

Conclusion

In this short article, we discuss how to check if a variable is set in PHP. We also looked at the differences between set, empty and null, as well as key considerations to keep in mind when using common functions isset(), is_null() and empty(). So the next time you need to check the status of the PHP variable, you will have all the information you need so that you can choose the right method and disambiguate all the ambiguity in your code.

FAQ for PHP variable checking (FAQ)

What is the purpose of the

function in PHP? isset()

The

function in PHP is used to determine whether the variable is set and not NULL. If the variable exists and is not NULL, this function returns true, otherwise false. This is a useful function to prevent errors when you try to access variables that may not exist or may have NULL values. isset() How is the difference between the

function and the

function in isset() in PHP? empty()

Although both

and isset() functions are used to check the values ​​of variables, they work slightly differently. The empty() function checks whether the variable is set and not NULL. On the other hand, the isset() function checks whether the variable is empty. If the variable does not exist or its value is equal to FALSE, the variable is considered empty. Therefore, if the variable is not set or its value is 0, 0.0, "", "0", NULL, FALSE, array(), or unset variable, empty() will return true. empty()

Can I use the

function to check if the array key exists? isset()

Yes, you can use the

function to check if there is a specific key in the array. If the key exists and its value is not NULL, isset() will return true. If the key does not exist or its value is NULL, isset() will return false. isset()

How to check if multiple variables are set in PHP?

You can check if multiple variables are set by passing multiple variables to the

function. This function returns true only if all specified variables are set and not NULL. isset()

What happens if I use

for unsetted variables? isset()

If you use

for unset variables, it will return false. This is because the isset() function in PHP is used to destroy the specified variable, so after the variable is unset, it is no longer set and its value is considered NULL. unset()

Can I use isset() to check if the variable is set to 0?

Yes, you can use isset() to check if the variable is set to 0. The isset() function only checks whether the variable is set and not NULL. It does not check the value of the variable. Therefore, if the variable is set to 0, isset() will return true.

How to use isset() to avoid undefined variable errors?

You can use isset() to check if the variable is set before trying to use it, thus avoiding undefined variable errors. If isset() returns false, it means the variable is not set and you can avoid using it, thus preventing undefined variable errors.

Can I use isset() with object properties in PHP?

Yes, you can use isset() with object properties in PHP. The isset() function can be used to check whether the properties exist in the object. If the property exists and its value is not NULL, isset() will return true.

What is the difference between isset() and is_null() in

?

The isset()is_null() function checks whether the variable is set and not NULL, while the isset() function checks whether the variable is NULL. So if the variable is set to NULL, is_null() will return false and

will return true.

isset() Can I use

to check if the form field has been filled in?

isset()Yes, you can use isset() to check if the form field has been filled in. When a form is submitted, the form field is sent to the server as a variable. You can use

to check if these variables are set, which indicates that the form field has been filled in.

The above is the detailed content of Quick Tip: How To Check if a Variable Is Set in PHP. For more information, please follow other related articles on the PHP Chinese website!