PHP中empty、isset、is_null差别
PHP中empty、isset、is_null区别
empty
如果 变量 是非空或非零的值，则 empty() 返回 FALSE。换句话说，""、0、"0"、NULL、FALSE、array()、var $var、未定义; 以及没有任何属性的对象都将被认为是空的，如果 var 为空，则返回 TRUE。
isset
如果 变量 存在(非NULL)则返回 TRUE，否则返回 FALSE(包括未定义）。变量值设置为：null，返回也是false;unset一个变量后，变量被取消了。注意，isset对于NULL值变量，特殊处理。
is_null
检测传入值【值，变量，表达式】是否是null,只有一个变量定义了，且它的值是null，它才返回TRUE . 其它都返回 FALSE 【未定义变量传入后会出错！】.
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What is the difference between null and NULL in c language
Sep 22, 2023 am 11:48 AM
The difference between null and NULL in C language is: null is a macro definition in C language, usually used to represent a null pointer, which can be used to initialize pointer variables, or to determine whether the pointer is null in a conditional statement; NULL is a macro definition in C language A predefined constant in , usually used to represent a null value, used to represent a null pointer, null pointer array or null structure pointer.
PHP determines whether a specified key exists in an array
Mar 21, 2024 pm 09:21 PM
This article will explain in detail how PHP determines whether a specified key exists in an array. The editor thinks it is very practical, so I share it with you as a reference. I hope you can gain something after reading this article. PHP determines whether a specified key exists in an array: In PHP, there are many ways to determine whether a specified key exists in an array: 1. Use the isset() function: isset($array["key"]) This function returns a Boolean value, true if the specified key exists, false otherwise. 2. Use array_key_exists() function: array_key_exists("key",$arr
What do undefined and null mean?
Nov 20, 2023 pm 02:39 PM
In JavaScript, both undefined and null represent the concept of "nothing": 1. undefined represents an uninitialized variable or a non-existent property. When a variable is declared but no value is assigned to it, the value of the variable is undefined , when accessing properties that do not exist in the object, the returned value is also undefined; 2. null represents an empty object reference. In some cases, the object reference can be set to null to release the memory it occupies.
When to use null and undefined
Nov 13, 2023 pm 02:11 PM
Both null and undefined indicate a lack of value or an undefined state. Depending on the usage scenario, there are some guiding principles for choosing to use null or undefined: 1. When you need to clearly indicate that a variable is empty or invalid, you can use null; 2. When a variable has been declared but not yet assigned a value, it will be set to undefined by default; 3. When you need to check whether a variable is empty or undefined, use the strict equality operator "===" to determine whether the variable is null or undefined. .
What is the difference between null and undefined
Nov 08, 2023 pm 04:43 PM
The difference between null and undefined is: 1. Semantic meaning; 2. Usage scenarios; 3. Comparison with other values; 4. Relationship with global variables; 5. Relationship with function parameters; 6. Nullability check; 7. Performance considerations; 8. Performance in JSON serialization; 9. Relationship with types. Detailed introduction: 1. Semantic meaning, null usually means knowing that this variable will not have any valid object value, while undefined usually means that the variable has not been assigned a value, or the object does not have this attribute; 2. Usage scenarios, etc.
What are the uses of null in java
Mar 01, 2024 am 10:10 AM
Usage: 1. Initialize the reference type variable to null, indicating that the variable does not currently point to any object; 2. Set the reference type variable to null, which can release the memory space of the object referenced by the variable and help the garbage collector to recover This object; 3. Use null to check whether a reference is empty. You can avoid the occurrence of NullPointerException by judging whether the reference is null. 4. Use null in conditional judgment to judge whether a reference is empty.
Detailed explanation of the meaning of true in PHP functions
Mar 16, 2024 am 08:57 AM
Detailed explanation of the meaning of true in PHP function In PHP, true is a Boolean value, representing the state of "true" or "yes". Using true in a function is usually used to express that a condition is true or to return a success status. This article will explain in detail the meaning of true in PHP functions from different perspectives, and give specific code examples. 1. Conditional judgment when true is returned: In PHP functions, true is usually used in conditional judgment to indicate that the condition is true. For example: functioni
What does true mean in PHP function?
Mar 16, 2024 pm 12:12 PM
Title: What does true mean in PHP functions? In PHP programming, true represents true in a Boolean value, usually indicating the state of "true". When true is used in a function, it is usually used to determine whether a condition is true or to perform some operation. The following will use specific code examples to illustrate what true means in PHP functions. example:
