PHP date and time application eight: add or subtract the number of days from a specific date

Declaring the location of a function in PHP is important because it affects the availability of the function. 1. Functions are most commonly declared in .php files and loaded through include or require when needed; 2. They can be placed on the top of the script or in a dedicated function file, as long as they are defined before calling, it is recommended to centrally manage to improve maintenance; 3. In object-oriented programming, functions can be declared as class methods or in namespace to avoid naming conflicts; 4. The same function cannot be declared repeatedly, and conflicts can be avoided through include_once, require_once or function_exists checks. Ensuring that the function is defined before being called and only once is the key to handling function declarations in PHP.

PHP ignores the execution overhead of comments, because comments are discarded during the compilation stage and will not enter the opcode execution process; 2. The only negligible performance impact is the microsecond parsing time when the script is first loaded, and there is almost no impact after OPcache is enabled; 3. Priority should be paid to the real performance bottlenecks such as database queries and loops, rather than the number of comments.

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In PHP development, function naming should start with a verb to maintain consistency, avoid blurred names, and control length. 1. Use clear verbs such as get, set, calculate, etc. to express behavioral intentions; 2. Use is, has, get and other prefixes for the return value; 3. Follow the project specifications, recommend camelCase to avoid confusing naming style; 4. Avoid abbreviation and vague names, and use complete words to improve readability; 5. The length of the function name is moderate, and it is recommended to be controlled within 3 to 5 words. Too long may require splitting responsibilities.

Common function usage errors in PHP development include: 1. Ignore the return value type and error handling, and check the return value and use strict comparison; 2. If the parameter order is wrong or the type does not match, you should consult the document and enable the type declaration; 3. Ignore the difference between reference passing and value passing, and confirm whether the original variable will be modified before use; 4. Confuse variadic function parameters and default parameters, and put the default parameters at the end and verify the variadic parameters.

Defining functions with optional parameters in PHP can be implemented through parameter default values. 1. Specify the default value for the parameter when defining the function. If it is not passed in during the call, the default value is used. Parameters with default values must be placed after the parameter without default value; 2. Default values can be set separately by multiple optional parameters. Parameters must be passed in sequence when calling, and intermediate parameters cannot be skipped; 3. When there are many parameters, parameters can be passed in arrays, and the default values and incoming values can be combined to improve flexibility and maintainability.

In PHP, the trim() function can remove whitespace characters at both ends of the string. If you need to remove non-whitespace characters, you can specify it through the second parameter. When only one-sided whitespace is removed, ltrim() or rtrim() can be used. To remove excess whitespace inside the string, you need to combine regular expressions to use the preg_replace() function. trim() removes spaces, tabs, newlines and empty bytes by default, and does not affect the content in the middle of the string. ltrim() is used to remove the left blank, rtrim() is used to remove the right blank, and the regular expression '/\s /' can match any consecutive whitespace characters and replace them with a single space to achieve internal whitespace cleaning.

The way to define mutable parameter functions in PHP is to use the... operator, which allows the function to accept any number of parameters. 1. Add... before function parameters, such as functionsum(...$numbers), and the parameters will be stored in an array. 2. Variable parameter functions are suitable for mathematical operations, string splicing, routing or event processing scenarios. 3. For versions before PHP5.6, variadic parameter behavior can be simulated through func_get_args(), func_num_args() and func_get_arg(). For example logMessages("Userloggedin","Sessio