My solutions for problems posted on https://codegolf.stackexchange.com/questions/tagged/code-golf

• use short names (1 letter) for variables, function names and function arguments;
• use short syntax for arrays; it requires PHP 5.4 or newer (saves 5 bytes for each array);
• combine initialization of many variables with the same value into a chained assignment; (saves at least 2 bytes for each variable);
• remove the initialization with 0, '' or array() completely when the first use of the variable is in the correct context; PHP triggers a notice and uses 0, '' or array() as default value when an undefined variable is used in a numeric, string or array operation; (saves 5-6 bytes for each variable);
• squeeze the initialization of variables, if possible, into the initialization expression of the for statement; (saves 1 byte);
• squeeze as much statements as possible from the for block inside the last expression of the for statement; (saves 1-2 bytes);
• squeeze, when possible, variable assignments inside their first usage (comparisons, when used as index etc); (saves at least 3 bytes);
• strip the quotes or apostrophes from around strings that can be used as constant names; when PHP encounters an identifier that looks like a constant name but no constant with that name is defined, it triggers a notice and converts the name into a string; (saves 2 bytes);
• strip the whitespace between a PHP keyword (as, return, else, echo etc) and the variable that follows it (saves 1 byte);
• strip the block markers ({ and }) when they enclose a single statement; (saves 2 bytes);
• combine multiple expression statements, if possible, into a single expression using the comma operator (,); it doesn't save any byte but allows removing the block markers when its outcome is a single expression inside a block (see above);
• extract duplicate string fragments into variables and use the variables inside double-quoted strings;
• extract into variables the names of functions that are invoked two or more times; don't quote the names (see above);