PHP coding specifications (22)

8.4 except for the variable name, all instances, including classes and class constants, use case-insensitive mixing. The first letter of the first word is lowercase, and the last word is capitalized. Variable names should not start with an underscore or dollar sign, although this is syntactically allowed. Variable names should be brief and descriptive. The variable name should be SyntaxHighlighter. all (); 8.4 variables

Except for the variable name, all instances, including class and class constants, are case-insensitive. The first letter of the first word is lowercase, and the first letter of the subsequent word is capitalized. Variable names should not start with an underscore or dollar sign, although this is syntactically allowed.
Variable names should be brief and descriptive. The selection of variable names should be easy to remember, that is, they can point out their purposes. Avoid the variable name of a single character unless it is a one-time temporary variable. Temporary variables are usually named I, j, k, m, and n. They are generally used for integer type; c, d, e, they are generally used for struct type.

Char $ c;
Int $ I;
Float $ myWidth;
8. instance variables

The case-sensitive rule is similar to the variable name, except for the preceding underline.

Int $ _ employeeId;
String $ _ name;
Customer $ _ customer;
8.6 constants

The declaration of class constants and ANSI constants should be in uppercase and words should be separated by underscores. (Avoid ANSI constants whenever possible, which may cause errors)

Static final int $ MIN_WIDTH = 4;
Static final int $ MAX_WIDTH = 999;
Static final int $ GET_THE_CPU = 1;