Summary of various PHP control statements_PHP tutorial

PHP control statement 1. IF statement

IF statement is an important feature in most languages. It is based on Conditional execution program segment. PHP's IF statement is similar to C:

1. if (expr)
2. statement

As discussed in Expression, expr is evaluated to its truth value. If expr is TRUE, PHP executes the corresponding statement, if it is FALSE it ignores it.
If $a is greater than $b, the following example will show 'a is bigger than b':

1. if ($a >$b)
2. print "a is bigger than b";

Often, you want to execute more than one statement based on a condition. Of course, there is no need to add an IF judgment to every statement. Instead, multiple statements can be grouped into a statement group.

If statements can be nested in other IF statements, allowing you to flexibly and conditionally execute various parts of the program.

PHP control statement 2. ELSE statement

Usually you want to execute one statement when a specific condition is met, and execute another statement when the condition is not met. ELSE is used to do this. ELSE extends the IF statement and executes another statement when the IF statement expression is FALSE. For example, if the following program is executed, if $a is greater than $b, it will display 'a is bigger than b', otherwise it will display 'a is

<ol class="dp-xml">
<li class="alt"><span><span>NOT bigger than b’:   </span></span></li>
<li>
<span>if ($a</span><span class="tag">></span><span>$b) {   </span>
</li>
<li class="alt"><span>print "a is bigger than b";   </span></li>
<li><span>}   </span></li>
<li class="alt"><span>else {   </span></li>
<li><span>print "a is NOT bigger than b";   </span></li>
<li class="alt"><span>}  </span></li>
</ol>

PHP control statement 3, ELSEIF statement

ELSEIF, as the name suggests, is a combination of IF and ELSE. Similar to ELSE, it extends the IF statement to execute other statements when the IF expression is FALSE. But unlike ELSE, it only executes other statements when the ELSEIF expression is also TRUE.

You can use multiple ELSEIF statements in one IF statement. The first statement whose ELSEIF expression is TRUE will be executed. In PHP 3, you can also write 'else if' (written as two words) and 'elseif' (written as one word) with the same effect. This is just a small difference in the way it's written (if you're familiar with C, it's the same), the result is exactly the same.
The ELSEIF statement is only executed when the IF expression and any preceding ELSEIF expression are both FALSE, and the current ELSEIF expression is TRUE.
The following is an IF statement in a nested format containing ELSEIF and ELSE:

<ol class="dp-xml">
<li class="alt"><span><span>if ($</span><span class="attribute">a</span><span>==5):   </span></span></li>
<li><span>print "a equals 5";   </span></li>
<li class="alt"><span>print "...";   </span></li>
<li>
<span>elseif ($</span><span class="attribute">a</span><span>==6):   </span>
</li>
<li class="alt"><span>print "a equals 6";   </span></li>
<li><span>print "!!!";   </span></li>
<li class="alt"><span>else:   </span></li>
<li><span>print "a is neither 5 nor 6";   </span></li>
<li class="alt"><span>endif;  </span></li>
</ol>

PHP control statement 4. WHILE statement

WHILE loop is a simple loop in PHP 3. Just like in C. The basic format of the WHILE statement is:

WHILE(expr) statement

The meaning of the WHILE statement is very simple. It tells PHP to execute the nested statement repeatedly as long as the WHILE expression is TRUE. The value of the WHILE expression is checked at the beginning of each loop, so even if its value is changed within the nested statement, this execution will not terminate until the end of the loop (each time PHP runs a nested statement is called a loop ). Similar to the IF statement, you can use curly braces to enclose a group of statements and execute multiple statements in the same WHILE loop:

WHILE(expr): statement ... ENDWHILE;

The following examples are exactly the same, they all type numbers 1 to 10:

/* example 1 */

<ol class="dp-xml">
<li class="alt"><span><span>$</span><span class="attribute">i</span><span>=</span><span class="attribute-value">1</span><span>;   </span></span></li>
<li><span>while ($i0); </span></li>
</ol>

The above loop is only executed once, because after the first loop, when true is checked value expression, it evaluates to FALSE ($i is not greater than 0) and loop execution terminates.

PHP control statement 5. FOR loop statement

The FOR loop is the most complex loop in PHP. Just like in C. The syntax of a FOR loop is:
FOR (expr1; expr2; expr3) statement
The first expression (expr1) is unconditionally evaluated (executed) at the beginning of the loop.
Every time through the loop, the expression expr2 is evaluated. If the result is TRUE, loops and nested statements continue to execute. If the result is FALSE, the entire loop ends.

At the end of each loop, expr3 is evaluated (executed). Each expression can be empty. If expr2 is empty, the number of loops is variable (PHP defaults to TRUE, like C). Don't do this unless you want to end the loop with a conditional BREAK statement in place of the FOR truth expression.
Consider the following example. They all display the numbers 1 to 10:

/* example 1 */

<ol class="dp-xml">
<li class="alt"><span><span> for ($</span><span class="attribute">i</span><span>=</span><span class="attribute-value">1</span><span>; $i10) {   </span></span></li>
<li><span>break;   </span></li>
<li class="alt"><span>}   </span></li>
<li><span>print $i;   </span></li>
<li class="alt"><span>}   </span></li>
<li><span>/* example 3 */   </span></li>
<li class="alt">
<span>$</span><span class="attribute">i</span><span> = </span><span class="attribute-value">1</span><span>;   </span>
</li>
<li><span>for (;;) {   </span></li>
<li class="alt">
<span>if ($i </span><span class="tag">></span><span>10) {   </span>
</li>
<li><span>break;   </span></li>
<li class="alt"><span>}   </span></li>
<li><span>print $i;   </span></li>
<li class="alt"><span>$i++;   </span></li>
<li><span>}  </span></li>
</ol>

Of course, the first example is obviously the best, but with this you can find out what is happening in the FOR loop Empty expressions can be used in many situations.
Other languages ​​have a foreach statement to iterate over an array or hash table. PHP uses while statements and list(), each() functions to achieve this function.

PHP control statement 6. SWITCH selection statement

The SWITCH statement is like a series of IF statements for the same expression. Many times, you want to compare the same variable (or expression) with many different values, and execute different program segments based on different comparison results. This is what the SWITCH statement is for.
The following two examples do the same thing in different ways, one using a set of IF statements and the other using a SWITCH statement:

/* example 1 */

<ol class="dp-xml">
<li class="alt"><span><span>if ($</span><span class="attribute">i</span><span> == 0) {   </span></span></li>
<li><span>print "i equals 0";   </span></li>
<li class="alt"><span>}   </span></li>
<li>
<span>if ($</span><span class="attribute">i</span><span> == 1) {   </span>
</li>
<li class="alt"><span>print "i equals 1";   </span></li>
<li><span>}   </span></li>
<li class="alt">
<span>if ($</span><span class="attribute">i</span><span> == 2) {   </span>
</li>
<li><span>print "i equals 2";   </span></li>
<li class="alt"><span>}   </span></li>
<li><span>/* example 2 */   </span></li>
<li class="alt"><span>switch ($i) {   </span></li>
<li><span>case 0:   </span></li>
<li class="alt"><span>print "i equals 0";   </span></li>
<li><span>break;   </span></li>
<li class="alt"><span>case 1:   </span></li>
<li><span>print "i equals 1";   </span></li>
<li class="alt"><span>break;   </span></li>
<li><span>case 2:   </span></li>
<li class="alt"><span>print "i equals 2";   </span></li>
<li><span>break;   </span></li>
<li class="alt"><span>} </span></li>
</ol>

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