How do I convert 12 hour clock time into 24 hour clock time in PHP?

From the docs for date(): The H format character gives the hour in 24h format. Also, you can use G if you do not want the leading 0 for hours before noon.

Examples (if current time was seven-something-AM)
date('H:i:s') -> "07:22:13"
date('G:i:s') -> "7:22:13"

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For your specific case:

$timestamp = date("d/m/Y H:i:s", time());

According to the manual the difference is in the capitalization of hour portion: "h" returns a 12-hour format of an hour with leading zeros, 01 through 12. "H" returns a 24-hour format of an hour with leading zeros, 01 through 23.