In-depth understanding of echo() and print(), require() and include() in php

$r = print "Hello World";

This means that print can be used in some complex expressions, but echo cannot. However, because the echo statement does not require any value to be returned, the echo statement in the code runs slightly faster than the print statement.

2. The difference between include and require include() and require() have basically the same function (include), but there are some differences in usage. include() is a conditional inclusion function, while require() is an unconditional inclusion function. For example, in the following code, if the variable $a is true, the file a.php will be included:

if($a){
include("a.php");
}

And require() is different from include(), no matter what value $a takes, The following code will include the file a.php into the file:

if($a){
require("a.php");
}

In terms of error handling, use the include statement. If an include error occurs, the program will skip include statement, although an error message will be displayed, the program will continue to execute! But require will give you a fatal error. Of course, we can also understand Qifen literally: require means a very strong request or request.

3, .require_once() and include_once() statements Simple require_once() and include_once() statements correspond to require() and include() statements respectively. The require_once() and include_once() statements are mainly used when multiple files need to be included, which can effectively avoid errors in repeated definitions of functions or variables caused by including the same piece of code.

4. The difference between empty string ('') and NULL Both empty strings and NULL in PHP are stored with a value of 0, but their types are different. You can try echo gettype(''); and echo gettype(NULL); and you will find that they print They are string and NULL respectively. Of course, 0 is also easy to confuse. You can try echo gettype(0); print the type and you will find that the type of 0 is integer (integer). You can see string (''), NULL and 0 is "equal" but not of equal type.

5. The difference between isset and empty We can understand from the literal meaning: empty is to determine whether a variable is "empty", while isset is to determine whether a variable has been set. But there is one thing you must pay attention to here: when the value of a variable is 0, empty considers the variable to be equal to empty, which is equivalent to no setting. For example, when we detect the $id variable, when $id=0, we use empty and isset to detect whether the variable $id has been configured. Both will return different values: empty thinks it is not configured, and isset can get the value of $id. , look at the example below:

$id=0;
emptyempty($id)?print "I am empty":print "I am $id ."; //Result: I am empty
!isset($id) ?print "I am empty":print "I am $id .";//Result: I am 0

6, == (equal) and === (equal) The difference Recall the difference between the fourth empty string ("") and NULL above, here is another example:

'' == NULL;
'' === NULL;

After running, the first one is true and the second one is false! It can be seen that == only compares whether the values ​​are equal, while === not only compares the values, but also compares the types, which is more strict.