Mathematical Italic Small Phi

It is a variable: a letter standing for a quantity. The formula tells you which quantity; phi has no single fixed meaning across fields. In LaTeX, a math typesetting system, \varphi requests the loopy form, while \phi traditionally requests the straight form.

The oldest surviving inscription in Greek letters is dated to about 740 BCE. As Greek writing adapted the Phoenician consonant script (a system that mainly wrote consonants), it added phi for /pʰ/, a p followed by a puff of breath. In Modern Greek, the letter is pronounced /f/. One hypothesis links phi’s form to qoppa, an older Greek letter, but other scholars reject that connection.

Can text cleanup turn 𝜑 into plain φ?

Yes; NFKC or NFKD can remove its math style. These text-cleanup forms fold mathematical alphabets back to ordinary Greek or Latin letters. For 𝜑, the result is φ, which can erase a distinction an equation relies on. NFC preserves the mathematical alphabet.