The easiest decision - making statement to understand is the if statement. The basic form of an if construct is as follows:

if ( expression ) {
// Run this code
}
// More code here

If the expression inside the parentheses evaluates to true , the code between the braces is run. If the expression evaluates to false , the code between the braces is skipped. That ’ s really all there is to it. It’s worth pointing out that any code following the closing brace is always run, regardless of the result of the test. So in the preceding example, if expression evaluates to true , both the Run this code and More code here lines are executed; if expression evaluates to false , Run this code is skipped but More code here is still run. Here ’ s a simple real - world example:

$widgets = 23;
if ( $widgets == 23 ) {
echo "We have exactly 23 widgets in stock!";
}

The first line of the script creates a variable, $widgets , and sets its value to 23 . Then an if statement uses the == operator to check if the value stored in $widgets does indeed equal 23 . If it does — and it should! — the expression evaluates to true and the script displays the message: " We have exactly 23 widgets in stock! " If $widgets doesn’t hold the value 23 , the code between the parentheses — that is, the echo() statement — is skipped. (You can test this for yourself by changing the value in the first line of code and re - running the example.) Here’s another example that uses the >= (greater than or equal) and <= (less than or equal) comparison operators, as well as the && (and) logical operator:

$widgets = 23;
if ( $widgets > = 10 & & $widgets < = 20 ) {
echo "We have between 10 and 20 widgets in stock.";
}

This example is similar to the previous one, but the test expression inside the parentheses is slightly more complex. If the value stored in $widgets is greater than or equal to 10 , and it ’ s also less than or equal to 20 , the expression evaluates to true and the message " We have between 10 and 20 widgets in stock. " is displayed. If either of the comparison operations evaluates to false , the overall expression also evaluates to false , the echo() statement is skipped, and nothing is displayed.

The key point to remember is that, no matter how complex your test expression is, if the whole expression evaluates to true the code inside the braces is run; otherwise the code inside the braces is skipped and execution continues with the first line of code after the closing brace.

You can have as many lines of code between the braces as you like, and the code can do anything, such as display something in the browser, call a function, or even exit the script. In fact, here ’ s the previous example rewritten to use an if statement inside another if statement:

$widgets = 23;
if ( $widgets > = 10 ) {
if ( $widgets < = 20 ) {
echo "We have between 10 and 20 widgets in stock.";
}
}

The code block between the braces of the first if statement is itself another if statement. The first if statement runs its code block if $widgets > = 10 , whereas the inner if statement runs its code block — the echo() statement — if $widgets < = 20 . Because both if expressions need to evaluate to true for the echo() statement to run, the end result is the same as the previous example.

If you only have one line of code between the braces you can, in fact, omit the braces altogether:

$widgets = 23;
if ( $widgets == 23 )
echo "We have exactly 23 widgets in stock!";

However, if you do this, take care to add braces if you later add additional lines of code to the code block. Otherwise, your code will not run as expected!