No matter how cautious we are when we write programs, it is inevitable to make mistakes. These errors usually confuse the PHP compiler. If developers cannot understand the meaning of the compiler error message, these error messages are not only useless, but often frustrating.
When compiling a PHP script, the PHP compiler does everything it can to report the first problem it encounters. This creates a problem: PHP can only recognize the error when it occurs (the problem is described in detail later in this article). It is precisely because of this that the compiler points out the line that makes the error, which may appear to be syntax correct, or may be a line that does not exist at all!
A better understanding of error messages can greatly save the time it takes to determine and correct error content. Therefore, in this article, I will strive to clarify many different types of PHP error information, and how to correctly understand the meaning of various error information during the development process.
What is described in this article has nothing to do with the version of PHP you are applying, 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 figure out why the compiler reports an error on a certain line, you must first clarify the mechanism for the compiler to parse PHP code. I do not intend to discuss this in detail in this article, but we will discuss some simple concepts that are more prone to errors.
Variable declaration If you declare a variable in a statement, the specific method is as follows:
$variable = 'value';
The compiler first finds the value of the right half of the statement (that is, everything to the right of the equal sign). In some programming books, denoted this as the RHS of the statement (right half). It is precisely this part of the statement that often causes errors. If the syntax used is incorrect, a parsing error will occur.
Parsing error
Parse error: parsing error, unexpected T_WHILE in c:\program files\apache group\apache\htdocs\ on line 19
Every time the previous error is determined, the parsing errors continue to appear one by one. Because PHP stops executing scripts after the first parsing error, debugging and correcting this series of errors can often make people feel particularly bored.
Moreover, parsing errors have very little information and hardly report the line number where the error is located. The specific reason is that when an error occurs, the compiler determines that the syntax of several lines should look valid until an invalid syntax is encountered. The most likely situation is that predefined words are used in the expression, for example;
while = 10; // Bad ? while is a predefined word that 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 have to do so, PHP will report more errors, which you can't stand.
Regarding this question, the following example might help you. Please consult and read the PHP code shown below:
<?php
$b = "somevalue"
if($b == "somevalue"){
print "Hello world!";
}
?>
The error is on the "$b=" line (the semicolon is missing at the end of the statement), so the error should be "parser error: the semicolon is missing on line 3" right? It should not be determined based on the parser:
Parse error: parse error, unexpected T_IF in c:\program files\apache
group\apache\htdocs\ on line 4
On line 4, the syntax of the if() statement is correct. So, what made the compiler confused? The clue is the "unexpected T_IF" part. When an "unexpected T_???" error appears, it means that the compiler finds that it appears where the predefined word should not appear. T_IF stands for if(), T_WHILE stands for while(), T_FOR stands for for(), etc.
Thankfully, some of the reasons for the errors are simple:
The statement does not end with a semicolon (;), such as the example above. Quotes are missing in the string. Some other common mistakes The most common mistake I've seen is the error that occurs when a function or a loop is not ended with braces ( } ), which is probably the most common and annoying error. The specific code is as follows: function UselessFunction() {
for($i < 0; $i < 10; $i++){
}
The following error will be generated:
Parse error: parse error, unexpected $ in c:\program files\apache
group\apache\htdocs\ on line 9
Since the function UselessFunction does not end with braces ( } ), the PHP compiler constantly searches for braces indicating the end until it reaches the end of the file. Because the compiler does not find a matching brace, an error will be reported at the end of the file.
If the hierarchy of the code is correctly reflected, the error message becomes very obvious. If the hierarchy of the code is not marked, it will become almost impossible to find out what you have forgotten in the end. So, remember to indicate the hierarchy of the code. The Tab key can easily achieve this. For subsequent developers, it will be easier to grasp the code framework and modify it.
MySQL Error
Another extremely annoying error message is the most common MySQL error, which often makes PHP newbies have a headache: Warning: Supplied argument is not a valid MySQL result resource in...
The line that is reported above may be: while($row = mysql_fetch_array($result)) { The parameter $result is not a valid resource. In English it means that mysql_fetch_array will not be processed because the query fails. The syntax of either query is invalid (you should copy-paste the query into the MySQL console reference for testing), or the connection to the database fails (in which case you should check the username and password, etc.).
Prevent errors from happening
In the first step, the smart coder can take the following steps to eliminate the following errors:
· At the end of each statement, you don't have to consider adding a semicolon - this should become a habit.
Always indicate the hierarchy of the code as much as possible, which allows you to see if you forget to add braces at places like if calls or function ends.
· Please use an editor that highlights syntax (such as HTML-Kit). With the help of this type of editor, you can determine whether you forgot to add quotes, missing semicolons, etc. in conclusion In this article, we have a certain understanding of some errors that may seem meaningless that the PHP compiler can report. We need to apply what we have learned to how to avoid errors and how to correct them when they occur. Debugging is one of the most important parts of all the work of a developer. Improving debugging efficiency can greatly speed up the progress of the entire work, shorten the time it takes to complete a project, and can also significantly reduce the mental stress caused by code failure.