Examples: Callback function examples A PHP function is passed by its name as a string. Any built-in or user-defined function can be used, except language constructs such as: array(), echo, empty(), eval(), exit(), isset(), list(), print or unset(). A method of an instantiated object is passed as an array containing an object at index 0 and the method name at index 1. Accessing protected and private methods from within a class is allowed. Static class methods can al

Examples: PHP does not require (or support) explicit type definition in variable declaration; a variable's type is determined by the context in which the variable is used. That is to say, if a string value is assigned to variable $var, $var becomes a string. If an integer value is then assigned to $var, it becomes an integer. An example of PHP's automatic type conversion is the addition operator '+'. If either operand is a float, then both operands are evaluated as

Examples: Associativity <?php $a = 3 * 3 % 5; // (3 * 3) % 5 = 4 // ternary operator associativity differs from C/C++ $a = true ? 0 : true ? 1 : 2; // (true ? 0 : true) ? 1 : 2 = 2 $a = 1; $b = 2; $a = $b += 3; // $a = ($b += 3) -> $a = 5, $b = 5 ?> Undefined order of evaluation Operator precedence and associativity only determine how expressions are grouped, they do not specify an order of evaluation. PHP does not (in the general

Examples: The division operator ("/") returns a float value unless the two operands are integers (or strings that get converted to integers) and the numbers are evenly divisible, in which case an integer value will be returned. Operands of modulus are converted to integers (by stripping the decimal part) before processing. The result of the modulus operator % has the same sign as the dividend — that is, the result of $a % $b will have the same sign as $a. For exam

Examples: The basic assignment operator is "=". Your first inclination might be to think of this as "equal to". Don't. It really means that the left operand gets set to the value of the expression on the right (that is, "gets set to"). The value of an assignment expression is the value assigned. That is, the value of "$a = 3" is 3. This allows you to do some tricky things: <?php $a = ($b = 4) + 5; // $a is equal to 9 now, and $b has been set to 4. ?>

Examples: Bit shifting in PHP is arithmetic. Bits shifted off either end are discarded. Left shifts have zeros shifted in on the right while the sign bit is shifted out on the left, meaning the sign of an operand is not preserved. Right shifts have copies of the sign bit shifted in on the left, meaning the sign of an operand is preserved. Use parentheses to ensure the desired precedence. For example, $a & $b == true evaluates the equivalency then the bitwise and

Examples: If you compare a number with a string or the comparison involves numerical strings, then each string is converted to a number and the comparison performed numerically. These rules also apply to the switch statement. The type conversion does not take place when the comparison is === or !== as this involves comparing the type as well as the value. <?php var_dump(0 == "a"); // 0 == 0 -> true var_dump("1" == "01"); // 1 == 1 -> true var_dump("10" == 

Examples: PHP supports one error control operator: the at sign (@). When prepended to an expression in PHP, any error messages that might be generated by that expression will be ignored. If you have set a custom error handler function with set_error_handler() then it will still get called, but this custom error handler can (and should) call error_reporting() which will return 0 when the call that triggered the error was preceded by an @. If the track_errors featu

Examples: PHP supports one execution operator: backticks (``). Note that these are not single-quotes! PHP will attempt to execute the contents of the backticks as a shell command; the output will be returned (i.e., it won't simply be dumped to output; it can be assigned to a variable). Use of the backtick operator is identical to shell_exec(). <?php $output = `ls -al`; echo "<pre>$output</pre>"; ?>

Examples: Here's a simple example script: <?php echo "<h3>Postincrement</h3>"; $a = 5; echo "Should be 5: " . $a++ . "<br />\n"; echo "Should be 6: " . $a . "<br />\n"; echo "<h3>Preincrement</h3>"; $a = 5; echo "Should be 6: " . ++$a . "<br />\n"; echo "Should be 6: " . $a . "<br />\n"; echo "<h3>Postdecrement</h3>"; $a = 5; echo "Should be 5: " . $a-- . "<br />\n"; echo "Should be 4: " . $a .