1. Built-in Functions

2. echo strrev("Hello world.");
Prints:
.dlrow 0lleH

3. echo str_repeat("Hip ", 2);
Prints:
Hip Hip

4. echo strtoupper("lowercase");
Prints:
LOWERCASE

5. str_replace("text_to_find", "text_to_replace_it_with", name_of_string_to_work_on);
$strWord="abcdefg";
$strWord=str_replace("a","aa",$strWord);
$strWord=str_replace("b","bb",$strWord);
$strWord=str_replace("c","cc",$strWord);
$strWord=str_replace("d","dd",$strWord);
$strWord=str_replace("e","ee",$strWord);
$strWord=str_replace("f","ff",$strWord);
$strWord=str_replace("g","gg",$strWord);
echo $strWord;
Prints:
aabbccddeeffgg

6. Good example: Formatting Output as Currency (Money)
Formatting Functions
output text with formatting applied to it using special formatting characters
Good example: Formatting Output as Currency (Money)

7. Time & Date Functions

8. checkdate(m, d, y)
determines if the date specified by a month, day, and year really exists on the calendar.
$valid=checkdate(13,1,1978);
if($valid)
echo "Real date";
else
echo "Not real date";

9. mktime
creates a timestamp for a given date and time. It takes as parameters the hour, minutes, seconds, month, day, and year.
echo mktime(0,0,0,1,1,2000); //create timestamp for given date
//number of hour 0-23
//number of minutes 0-59
//number of seconds 0-59
//number of month 1-12
//number of day 1-31
//year
The timestamp, used alone, is, as you can see, not all that useful. You can, however use it along with another function called date.

10. Leave this off to use current date/time
Format: date($format, $timestamp);
Example:
$timestamp= mktime(0,0,0,1,1,2000); //create timestamp for given date
echo date(SOME_FORMAT_YOU_SPECIFIED,$timestamp)."<br />";
Leave this off to use current date/time

11. Format
Description
Returned Value
DAY SPECIFIERS d
Day of month 2 digit, leading zeroes
01 to 31 D
Day of week, three letters
Mon to Sun j
Day of month, no lead zeroes
1 to 31 l
Day of week, full names
Sunday to Saturday N
Day of week, numeric, Monday to Sunday
1 to 7 S
Suffix for day o fmonth
st,nd, rd, or th w
Day of week, numeric, Sunday to Sat
0 to 6 z
Day of year
0 to 365
WEEK SPECIFIER W
Week number of year
1 to 52
MONTH SPECIFIERS F
Month name
January to December m
Month number with lead zeroes
01 to 12 M
Month name, three letters
Jan to Dec n
Month number, no lead zeroes
1 to 12 t
Number of days in given month
28, 29, 30, or 31
YEAR SPECIFIERS L
Leap year
1=yes, 0=no Y
Year, 4 digits
0000 to 9999 y
Year, 2 digits
00 to 99
TIME SPECIFIERS a
Before/after midday, lowercase
am, pm A
Before, after midday, uppercase
AM, PM g
Hour of day, 12-hr, no lead zeroes G
Hour of day, 24-hr, no lead zeroes
1 to 24 h
Hour of day, 12, lead zeroes H
Hour of day, 24, lead zeroes
01 to 24 i
Minutes with lead zeroes
00 to 59 s
Seconds, with lead zeroes

12. $timestamp= mktime(6,0,0,7,14,1978); $format="D, F j, Y";
Example
$timestamp= mktime(6,0,0,7,14,1978);
$format="D, F j, Y";
echo "I was born " . date($format,$timestamp);

13. $right_now=date("L");
if($right_now==1)
echo "This is a leap year";
else
echo "This is not a leap year.";

14. More Info