A constant is an identifier name for a simple value.
A constant is case-sensitive by default. By convention, constant identifiers are always uppercase. The name of a constant follows the same rules as any label in PHP. A valid constant name starts with a letter or underscore, followed by any number of letters, numbers, or underscores.
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The if construct is one of the most important features of many languages, PHP included. It allows for conditional execution of code fragments. PHP features an if structure that is similar to that of C:. The expression is evaluated to its Boolean value.
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Like else , it extends an if statement to execute a different statement in case the original if expression evaluates to FALSE. However, unlike else , it will execute that alternative expression only if the elseif conditional expression evaluates to TRUE. See Operator Precedence.
The switch statement is similar to a series of IF statements on the same expression. In many occasions, you may want to compare the same variable or expression with many different values, and execute a different piece of code depending on which value it equals to. This is exactly what the switch statement is for.
PHP Quick Guide: The Fundamentals
They behave just like their C counterparts. The basic form of a while statement is:. The meaning of a while statement is simple. The value of the expression is checked each time at the beginning of the loop, so even if this value changes during the execution of the nested statement s , execution will not stop until the end of the iteration each time PHP runs the statements in the loop is one iteration. They behave like their C counterparts. The syntax of a for loop is:. The first expression expr1 is evaluated executed once unconditionally at the beginning of the loop.
In the beginning of each iteration, expr2 is evaluated. If it evaluates to TRUE , the loop continues and the nested statement s are executed.
At the end of each iteration, expr3 is evaluated executed. Each of the expressions can be empty or contain multiple expressions separated by commas. In expr2 , all expressions separated by a comma are evaluated but the result is taken from the last part. The foreach construct provides an easy way to iterate over arrays. There are two syntaxes:. It is possible to customize object iteration. In PHP 5, when foreach first starts executing, the internal array pointer is automatically reset to the first element of the array.
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This means that you do not need to call reset before a foreach loop. As foreach relies on the internal array pointer in PHP 5, changing it within the loop may lead to unexpected behavior. In that case the value will be assigned by reference. Continue is used within looping structures to skip the rest of the current loop iteration and continue execution at the condition evaluation and then the beginning of the next iteration. Note : In PHP the switch statement is considered a looping structure for the purposes of continue.
If a switch is inside a loop, continue 2 will continue with the next iteration of the outer loop. The default value is 1 , thus skipping to the end of the current loop. Break ends execution of the current for , foreach , while , do-while or switch structure. The default value is 1 , only the immediate enclosing structure is broken out of.
This implementation defines the details of how array data is actually stored in memory, how arrays behave, how they can be accessed etc. Any valid PHP code may appear inside a function, even other functions and class definitions.
Function names follow the same rules as other labels in PHP. A valid function name starts with a letter or underscore, followed by any number of letters, numbers, or underscores. See also the Userland Naming Guide. Functions need not be defined before they are referenced, except when a function is conditionally defined as shown in the two examples below.
Information may be passed to functions via the argument list, which is a comma-delimited list of expressions. The arguments are evaluated from left to right.
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The representation of the language is based on the ANSI standard and includes extensions introduced in An index is included to help you quickly find the information you need.