Common error messages and explanations in PHP program compilation_PHP tutorial

When writing a program, no matter how careful you are, mistakes are inevitable. These errors usually confuse the PHP compiler. Compiler error messages are not only useless but often frustrating if developers don't understand what they mean.
　
When compiling a PHP script, the PHP compiler will do its best to report the first problem it encounters. This creates a problem: PHP can only identify the error when it occurs (this problem is described in detail later in this article). It is for this reason that the compiler points out that the line in error may appear to be syntactically correct on the surface, or it may be a line that does not even exist! and the time it takes to correct erroneous content. Therefore, in this article, I will try to clarify the many different types of PHP error messages and how to correctly understand the meaning of various error messages during the development process.
　
The content described in this article has nothing to do with the version of PHP you are using, because the various errors described in this article are not limited to specific errors in a particular version. In addition, we assume that you are a junior or intermediate programmer and have been engaged in programming for half a year or a year.
　
　How the compiler works
　
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. I'm not going to go into detail about this in this article, but we will discuss some simple concepts that are more likely to lead to errors.
　
　Variable declaration
　
　If you declare a variable in a statement, the specific method is as follows:
　
　$variable = 'value';
　
　Compiler First find the value of the right half of the statement (that is, everything to the right of the equal sign). In some programming books, this is represented as the RHS (right half) of the statement. It is this part of the statement that often causes errors. If you use incorrect syntax, you will get a parsing error.
　
　Parse error
　
　Parse error: Parse error, unexpected T_WHILE in c://program files//apache group//apache//htdocs//script.php on line 19

Parse errors keep appearing one after another every time the previous error is determined. Because PHP stops executing the script after the first parsing error, debugging and correcting this series of errors can often be particularly tiresome.
　
 Moreover, parsing errors have very little information, and the line number where the error occurs is almost never reported. The specific reason is that when an error occurs, the compiler determines that the syntax of several lines should look valid until it encounters invalid syntax. The most likely case is that predefined words are used in the expression, such as;

　while = 10; // Bad? while is a predefined word and cannot be assigned to a value
　
　Predefined words include while, function, etc., if PHP uses uses to evaluate your code. You cannot use these predefined words to name variables, and if you do, PHP will throw more errors than you can tolerate.
　
　 Regarding this issue, the following example may be helpful to you. Please read the PHP code shown below:
　
　$b = somevalue
　
　if($b == somevalue){
　
　print Hello world!;

　}
　
　?>
　
　The error is located in $b = line (a semicolon is missing at the end of the statement), so the error should be a parsing error: a semicolon is missing in line 3, right? ?It should not be judged based on the parser:
　
　Parse error: parse error, unexpected T_IF in c://program files//apache
　
　group//apache//htdocs//ereg2 .php on line 4
　
　In line 4, the syntax of the if() statement is correct. So, what is confusing the compiler? The clue is the unexpected T_IF part. When an unexpected T_??? error occurs, it means: the compiler found that a predefined word appears in a position where it should not appear. T_IF represents if(), T_WHILE represents while(), T_FOR represents for(), etc.
　
　Thankfully, the cause of some errors is also very simple:
　
　The statement is not ended with a semicolon (;), such as the example above. Quotation marks are missing in the string.
　
Some other common mistakes

The most common mistake I see is when closing a function or a loop without using braces (}), which is most likely The most common and annoying mistake.
　
　The specific code is as follows:
　
　function UselessFunction() {
　
　for($i < 0; $i < 10; $i++){
　
　}
　
　The following error will be generated:
　
　Parse error: parse error, unexpected $ in c://program files//apache
　
　group//apache//htdocs //ereg2.php on line 9
　
Since the function UselessFunction does not end with a brace ( } ), the PHP compiler keeps looking for the closing brace until it reaches the end of the file. 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. If the hierarchical structure of the code is not marked, it will be almost impossible to find out what has been forgotten in the end. So, remember, be sure to indicate the hierarchy of your code. The Tab key makes this easy. It will also be easier for subsequent developers to grasp the code framework and modify it.
　
　MySQL Error
　
　Another extremely annoying error message is the most common MySQL error, which often causes headaches for new PHP users: Warning: Supplied argument is not a valid MySQL result resource in...
　
　The wrong line reported above may be:
　
　while($row = mysql_fetch_array($result)) {
　
　The parameter $result is not An effective resource. In English it means that mysql_fetch_array cannot be processed because the query failed. Either the query has invalid syntax (you should copy-paste the query into the MySQL console reference to test), or the connection to the database failed (in which case you should double-check the username, password, etc.).
　
　Prevent errors from occurring
　
　In the first step, the smart coder can take the following steps to eliminate the following errors:
　
　· At the end of each statement, do not consider Add semicolons – this should become a habit.
　
　· Always indicate the hierarchy of your code whenever possible. This will allow you to see if you forgot to add braces at places like if calls or at the end of functions.
　
　· Please use an editor with syntax highlighting (such as HTML-Kit). With the help of such an editor, you can determine if you forgot to add a quote, if you are missing a semicolon, etc.

http://www.bkjia.com/PHPjc/371460.htmlwww.bkjia.comtruehttp: //www.bkjia.com/PHPjc/371460.htmlTechArticleWhen writing a program, no matter how careful you are, mistakes are always inevitable. These errors usually confuse the PHP compiler. If developers cannot understand the meaning of compiler error messages, then...