Through learning the PHP language, you should know that it is a function-based HTML scripting language. A huge function library supports the implementation of PHP language functions. Below we will introduce to you the related usage of PHP functions isset() and empty().
PHP’s isset() function is generally used to detect whether a variable is set
Format: bool isset (mixed var [, mixed var [, ...]])
Function: detect whether a variable is set
Return value:
If the variable does not exist, it returns FALSE
If the variable exists and its value is NULL, it also returns FALSE
If the variable exists and its value is not NULL, it returns TRUE
When checking multiple variables at the same time, each single item matches the previous one Returns TRUE only when requested, otherwise the result is FALSE
Versions: PHP 3, PHP 4, PHP 5
More instructions:
After using unset() to release a variable, it will no longer be isset().
PHP function isset() can only be used for variables, passing any other parameters will cause parsing errors.
To detect whether a constant has been set, use the defined() function.
PHP's empty() function determines whether the value is empty
Format: bool empty (mixed var)
Function: Check whether a variable is empty
Return value:
If the variable does not exist, it returns TRUE
If If the variable exists and its value is "", 0, "0", NULL, FALSE, array(), var $var; and an object without any attributes, TRUE is returned. If the variable exists and its value is not "", 0 , "0", NULL, FALSE, array(), var $var; and objects without any attributes, return FALSE
Versions: PHP 3, PHP 4, PHP 5
empty() can only be used for variables. Passing any other parameters will cause a Paser error and terminate the operation.
To detect whether a constant has been set, use the defined() function.
<?php
$var = 0;
// 结果为 true，因为 $var 为空
if (empty($var)) {
echo '$var is either 0 or not set at all';
}
// 结果为 false，因为 $var 已设置
if (!isset($var)) {
echo '$var is not set at all';
}
?>Note: Since this is a language structure rather than a function, it cannot be called by variable functions.
Note: empty() only detects variables, detecting anything that is not a variable will result in a parsing error. In other words, the following statement will not work: empty(addslashes($name)).The following is a code for a detailed example of isset and empty functions that we have tested. After reading this, it is basically the same:
<?php
error_reporting(E_ALL);
echo '<B>未定义$var</b><Br>';
echo "isset测试:<Br>";
if ( isset ( $var ))
{
echo '变量$var存在!<Br>' ;
}
echo "empty测试:<Br>";
if ( empty ( $var )){
echo '变量$var的值为空<Br>';
}
else
{
echo '变量$var的值不为空<Br>';
}
echo "变量直接测试:<Br>";
if ( $var ){
echo '变量$var存在!<Br>';
}
else {
echo '变量$var不存在!<Br>';
}
echo '----------------------------------<br>';
echo '<B>$var = \'\'</b><Br>';
echo "isset测试:<Br>";
$var = '';
if ( isset ( $var ))
{
echo '变量$var存在!<Br>' ;
}
echo "empty测试:<Br>";
if ( empty ( $var )){
echo '变量$var的值为空<Br>';
}
else
{
echo '变量$var的值不为空<Br>';
}
echo "变量直接测试:<Br>";
if ( $var ){
echo '变量$var存在!<Br>';
}
else {
echo '变量$var不存在!<Br>';
}
echo '----------------------------------<br>';
echo '<B>$var = 0</b><Br>';
echo 'isset测试:<Br>';
$var = 0 ;
if ( isset ( $var ))
{
echo '变量$var存在!<Br>' ;
}
echo "empty测试:<Br>";
if ( empty ( $var )){
echo '变量$var的值为空<Br>';
}
else
{
echo '变量$var的值不为空<Br>';
}
echo "变量直接测试:<Br>";
if ( $var ){
echo '变量$var存在!<Br>';
}
else {
echo '变量$var不存在!<Br>';
}
echo '----------------------------------<br>';
echo '<B>$var = null</b><Br>';
echo 'isset测试:<Br>';
$var = null ;
if ( isset ( $var ))
{
echo '变量$var存在!<Br>' ;
}
echo "empty测试:<Br>";
if ( empty ( $var )){
echo '变量$var的值为空<Br>';
}
else
{
echo '变量$var的值不为空<Br>';
}
echo "变量直接测试:<Br>";
if ( $var ){
echo '变量$var存在!<Br>';
}
else {
echo '变量$var不存在!<Br>';
}
echo '----------------------------------<br>';
echo '<B>$var ="php"</b><Br>';
echo 'isset测试:<Br>';
$var = "php";
if ( isset ( $var ))
{
echo '变量$var存在!<Br>' ;
}
echo "empty测试:<Br>";
if ( empty ( $var )){
echo '变量$var的值为空<Br>';
}
else
{
echo '变量$var的值不为空<Br>';
}
echo "变量直接测试:<Br>";
if ( $var ){
echo '变量$var存在!<Br>';
}
else {
echo '变量$var不存在!<Br>';
}
?>$id=0; empty($id)?print "It's empty .":print "It's $id ."; //结果：It's empty . print "<br>"; !isset($id)?print "It's empty .":print "It's $id ."; //结果：It's 0 .
This means that when we use the variable processing function, when the variable may have a value of 0, be careful when using empty(). It is more sensible to replace it with isset at this time.
When the URL tail parameter of a php page appears id=0 (for example: test.php?id=0), try to compare:if(empty($id)) $id=1; - 若 id=0 ，id 也会为1 if(!isset($id)) $id=1; - 若 id=0 ，id 不会为1
You can run the following code separately to detect the above inference:
if(empty($id)) $id=1; print $id; // 得到 1 if(!isset($id)) $id=1; print $id; //得到 0
If the variable is not Definition, empty() will return TRUE, isset() will return FLASE;
If var is a non-empty or non-zero value, then empty() returns FALSE. In other words, "", 0, "0", NULL, FALSE, array(), var $var; and objects without any properties will be considered empty, and TRUE will be returned if var is empty.
If a variable has been released using unset(), it will no longer be isset(). If you use isset() to test a variable that is set to NULL, it will return FALSE. Also note that a NULL byte ("?") is not equivalent to PHP's NULL constant.
Warning: isset() can only be used with variables, as passing any other parameters will cause a parsing error. If you want to check whether a constant has been set, you can use the defined() function.
When you want to judge whether a variable has been declared, you can use the isset function.
When you want to judge whether a variable has been assigned data and is not empty, you can use the empty function.
When you want to judge whether a variable exists and is not empty, use the isset function first and then use it. empty function
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