The difference between echo and print, require and include and other easily confused functions in require PHP

1. The difference between echo and print
The functions of echo and print in PHP are basically the same (output), but there are still subtle differences between the two. There is no return value after echo output, but print has a return value, and it returns false when its execution fails. Therefore, it can be used as a normal function. For example, after executing the following code, the value of variable $r will be 1.
$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 the same as include() Different, 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 include statement, if an include error occurs, the program will skip the include statement. Although the 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
Off topic, because they look similar, 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 the error of repeated definition of functions or variables caused by including the same piece of code.
4. The difference between empty string ('') and NULL
In PHP, both empty string and NULL are stored with a value of 0, but their types are different. You can try echo gettype(''); and echo gettype(NULL); You will find that what they print out are string and NULL respectively. Of course, 0 is also easy to confuse. You can try echo gettype(0); to print the type, you will find that the type of 0 is integer (integer). type), it can be seen that string (''), NULL and 0 are "equal values" but not equal types.
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, that is, it 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. , see 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. The difference between == (equal) and === (equal)
Review the fourth empty item above The difference between string ("") and NULL, let's look at another example:
'' == NULL;
'' === NULL;
After running, you will find that 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.

The above has introduced the differences between echo and print, require and include and other easily confused functions in require PHP, including the require content. I hope it will be helpful to friends who are interested in PHP tutorials.