How to troubleshoot and resolve php sql errors

There will be various types of errors in the interaction between PHP and SQL, such as syntax errors, logic errors, connection errors, etc. During the development process, we may encounter various error reports, which need to be investigated and resolved in a targeted manner.

Syntax errors refer to problems caused by errors in PHP or SQL syntax. If an undefined function name or variable appears in the code, a syntax error will occur.

The following is an example:

<?php
$my_array = array( 0=>"PHP", 1=>"HTML", 2=>"CSS" );
echo $my_array[3];
?>

In the above code, we think that there are three elements 0, 1, and 2 in the array, so we can use $my_array[3] to get the Four elements, but in fact there are only three elements in the array, so the code will have the following error:

PHP Notice: Undefined offset: 3 in test.php on line 3

Because we did not handle this error reasonably in the code, it will affect our normal program logic . Therefore, we need to deal with this grammatical error promptly.

Logic error refers to an error in the logic of the program, causing the program to fail to execute as expected. For example, if data should be inserted into the database, but the data insertion fails, the problem may be that the field types do not match, the data is empty, the data is duplicated, etc.

The following is an example:

<?php
$mysqli=mysqli_connect("localhost", "root", "", "test");
if (mysqli_connect_errno()){
    echo "Failed to connect to MySQL: ". mysqli_connect_error();
}

mysqli_query($mysqli,"INSERT INTO persons (FirstName, LastName, Age) VALUES (&#39;Peter&#39;, &#39;Griffin&#39;,30)");
mysqli_query($mysqli,"INSERT INTO persons (FirstName, LastName, Age) VALUES (&#39;Peter&#39;, &#39;Griffin&#39;,30)");
mysqli_query($mysqli,"INSERT INTO persons (FirstName, LastName, Age) VALUES (&#39;Peter&#39;, &#39;Griffin&#39;,&#39;thirty&#39;)");

mysqli_close($mysqli);
?>

In the above code, we try to insert the information of three people into the table named persons through mysqli_query. The first insert will succeed, the second one will fail because we're inserting the same data (the limit of a unique index), the third one will fail because we're inserting age as "thirty", and The type of the age field in the table is integer. Therefore, the following error will appear in the code:

PHP Warning:  mysqli_query(): Couldn't fetch mysqli in test.php on line 7
PHP Warning:  mysqli_query(): Couldn't fetch mysqli in test.php on line 8
PHP Warning:  mysqli_query(): Couldn't fetch mysqli in test.php on line 9

Because we do not handle these errors reasonably in the code, they will affect our normal program logic. Therefore, when performing database operations, we need to deal with possible logical errors.

Connection error refers to an error in the connection between PHP and SQL, resulting in the failure to establish a connection. For example, our SQL server fails, the network connection fails, the firewall blocks the connection, etc.

The following is an example:

<?php
$con=mysqli_connect("localhost","root","","test");
// 检查连接是否有效
if (mysqli_connect_errno()){
    echo "Failed to connect to MySQL: " . mysqli_connect_error();
}

mysqli_query($con,"INSERT INTO persons (FirstName, LastName, Age) VALUES (&#39;Mary&#39;, &#39;Moe&#39;, 20)");
mysqli_query($con,"INSERT INTO persons (FirstName, LastName, Age) VALUES (&#39;Sue&#39;, &#39;Anderson&#39;, 25)");
mysqli_query($con,"INSERT INTO persons (FirstName, LastName, Age) VALUES (&#39;Peter&#39;, &#39;Jones&#39;, 30)");

mysqli_close($con);
?>

In the above code, we try to connect to the database. If the connection fails, the following error will appear:

Failed to connect to MySQL: Can't connect to MySQL server on 'localhost'

Since we are Without reasonable processing in the code, we cannot establish a connection with the database. This error will cause the program to be unable to perform effective operations, so we need to handle this connection error in a timely manner.

In summary, there will be various types of errors in the interaction between PHP and SQL. For each error, we need to handle it accordingly in the code to ensure the stability and security of the program.

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