Inventory of common mistakes in PHP programming

　Overview: This article takes stock of the small mistakes and errors that PHP developers tend to overlook or not pay attention to when coding.

　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 continue to appear 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, 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, if you do not use curly brackets (}) to end a function or a loop, for example: function UselessFunction(){for($i

　　MySQL Error

　 Another type of error message is a MySQL error, which often causes a headache for PHP newbies, such as: Warning: Supplied argument is not a valid MySQL result resource in... The wrong line reported above It may be: while($row=mysql_fetch_array($result)){}The parameter $result is not a valid resource, because the query fails and mysql_fetch_array cannot 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.

　Pay attention to the difference between echo and print

　 Echo and print in PHP are both output functions, 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, 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.

　Pay attention to the difference between empty string ('') and NULL

　In PHP, empty strings and NULL are both stored with a value of 0, but their types are different. The former is string, while the latter is NULL, visible characters String ('') and NULL values ​​are equal but have different types.

Distinguish the difference between == (equal) and === (all equal)

Both are comparison operators, == (equal) only compares whether the values ​​are equal, while === (all equal) not only compares values Whether they are equal or not, the types are also compared for equality, which is more strict.

　Clear the difference between include and require

　The functions of include() and require() are basically the same, 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(), regardless of $ 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 an inclusion error occurs, the program will skip include statement, although an error message will be displayed, the program will continue to execute. However, the require statement prompts a fatal error.

　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.

　Clear 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 pointer operator ->.