Small PHP mistakes that are easy to make and their corresponding analysis

When we write programs, even experienced programmers will inevitably make mistakes. However, if the developer cannot notice these errors and cannot understand the meaning of the compiler error message, then these error messages are not only useless, but also often frustrating.
Better understanding of error messages can significantly save time spent on finding and correcting erroneous content. Therefore, in this article, we will introduce the common mistakes made when writing PHP programs, as well as different types of PHP error messages and their meanings. In addition, the content described in this article has nothing to do with the version of PHP used. For more PHP learning content, you can refer to "How to Give Good Names to Variables in Programs", "24 Useful PHP Class Libraries" and "Detailed Interpretation of PHP Arrays".
To understand why the compiler reports an error on a certain line, you must first understand the mechanism by which the compiler parses PHP code.
Variable declaration
If you declare a variable in a statement, as follows:
$var = 'value';
The compiler will first find the value of the right half of the statement, and it is precisely this part of the statement that often causes errors . If you use incorrect syntax, you will get a parsing error.

Parse error
For example, Parse error: Parse error, unexpected T_WHILE in c:program filesapache groupapachehtdocsscript.php on line 19
Every time the previous error is determined, parse errors keep appearing one after another, PHP in the first one Stop executing the script after parsing the error. Also, parsing errors have very little information, with almost no reporting of the line number where the error occurred. For example, if a predefined keyword is used in the expression, for example: while = 10; while is a predefined keyword and cannot be assigned a value. Predefined keywords include while, function, etc. We cannot use these predefined keywords to name variables, otherwise the compiler will report an error. Among them, T_IF represents if(), T_WHILE represents while(), T_FOR represents for(), etc.

Common Errors
There are also some common errors, such as statements not ending with a semicolon (;), missing quotation marks in the string, etc. In addition, there is no use of curly brackets ( } ) to end a function or a loop, for example:
function UselessFunction() {
  for($i }
will produce the following Error:
Parse error: parse error, unexpected $ in c:program filesapache groupapachehtdocsereg2.php on line 9
Since the function UselessFunction does not end with a brace (}), the PHP compiler will continue to look for the closing brace until it reaches the file until the end. Because the compiler doesn't find a matching brace, it reports an end-of-file error.
If the hierarchy of the code is correctly reflected, error messages will become very obvious. Otherwise, the code will be very difficult to debug. Therefore, be sure to indicate the hierarchical structure of the code, which will make it easier for subsequent developers to improve the code.

MySQL Error
Another type of error message is MySQL error, which often causes PHP novices to feel quite a headache, such as:
Warning: Supplied argument is not a valid MySQL result resource in...
The error reported above A line may be:
while($row = mysql_fetch_array($result)) {}
Parameter $result is not a valid resource, because the query fails and mysql_fetch_array will not be processed. If the syntax of any query is invalid or the connection to the database fails, you should go to the MySQL console for testing.

More common mistakes

1. Pay attention to the difference between echo and print
In PHP, echo and print are both used for 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 an ordinary function, for example, if $r = print "Hello World"; is executed, the value of variable $r will be 1. Moreover, the echo statement in the code runs slightly faster than the print statement.

2. Pay attention to the difference between empty string ('') and NULL
In PHP, empty string and NULL are both stored with a value of 0, but their types are different. The former is string, while the latter is NULL, which can be seen String ('') and NULL values ​​are equal but have different types.

3. Distinguish the difference between == (equal AND) and === (congruent AND)
Both are comparison operators, == (equal) only compares whether the values ​​are equal, while === (congruent AND) It not only compares whether the values ​​are equal, but also compares whether the types are equal, which is more strict.

4. Distinguish the difference between include and require
include() and require() have basically the same functions, 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 $a takes, the following code will include the file a.php:
if($a){
require("a.php");
}
In terms of error handling, use the include statement, if 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. However, the require statement prompts a fatal error.

5. Pay attention to the difference between isset and empty
empty is to determine whether a variable is "empty", while isset is to determine whether a variable has been set.

6. Distinguish the difference between self :: and this-->
When accessing member variables or methods in a PHP class, if the referenced variable or method is declared as const (constant) or static (static property), Then you must use the domain operator::, and if the referenced variable or method is not declared as const or static, then use the pointing operator ->.